Laser illumination device

ABSTRACT

An Electrically Switchable Bragg Grating (ESBG) despeckler device comprising at least one ESBG element recorded in a hPDLC sandwiched between transparent substrates to which transparent conductive coatings have been applied. At least one of said coatings is patterned to provide a two-dimensional array of independently switchable ESBG pixels. Each ESBG pixel has a first unique speckle state under said first applied voltage and a second unique speckle state under said second applied voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference in its entirety a PCTUS2006/043938 filed 13 Nov. 2006, claiming priority to U.S. provisionalpatent application 60/789,595 filed on 6 Apr. 2006, entitled METHOD ANDAPPARATUS FOR PROVIDING A TRANSPARENT DISPLAY.

This application incorporates by reference in their entireties U.S.provisional patent applications:

Ser. No. 60/935,109 filed 26 Jul. 2007, entitled LASER DISPLAYS;Ser. No. 61/071,230 filed 18 Apr. 2008, entitled SCROLLING ILLUMINATOR;Ser. No. 61/071,229 filed 18 Apr. 2008, entitled SCROLLING FLAT PANELDISPLAY; andSer. No. 61/071,232 filed 18 Apr. 2008 DUAL SCREEN PROJECTION DISPLAY33.This application incorporates by reference in its entirety the UnitedKingdom patent application GB0718706.5 filed 25 Sep. 2007.

BACKGROUND OF THE INVENTION

The present invention relates to an illumination device, and moreparticularly to a laser illumination device based on electricallyswitchable Bragg gratings.

Miniature solid-state lasers are currently being considered for a rangeof display applications. The competitive advantage of lasers in displayapplications results from increased lifetime, lower cost, higherbrightness and improved colour gamut. As lasers are polarized, they areideally suited to Liquid Crystal on Silicon (LCoS) or High TemperaturePoly Silicon (HTPS) projectors. In contrast to incoherent sources,lasers do not result in light from unwanted polarization states beingdiscarded.

Laser displays suffer from speckle, a sparkly or granular structure seenin uniformly illuminated rough surfaces. Speckle arises from the highspatial and temporal coherence of lasers. Speckle reduces imagesharpness and is distracting to the viewer.

Several approaches for reducing speckle contrast have been proposedbased on spatial and temporal decorrelation of speckle patterns. Moreprecisely, speckle reduction is based on averaging multiple (M) sets ofspeckle patterns from a speckle surface resolution cell with theaveraging taking place over the human eye integration time. The speckleresolution cell is essentially the smallest area of the image that theeye can resolve. Under optimal conditions speckle contrast is reducesfrom unity to the square root of M. The value of M should be as large aspossible. However, the value of M is limited by the numerical apertureof the imaging optics. In other words the minimum cell size isapproximately equal to the laser wavelength divided by the numericalaperture.

Speckle may be characterized by the parameter speckle contrast which isdefined as the ratio of the standard deviation of the speckle intensityto the mean speckle intensity. Temporally varying the phase patternfaster than the eye temporal resolution destroys the light spatialcoherence, thereby reducing the speckle contrast.

The basic statistical properties of speckle are discussed by J. W.Goodman in a first paper entitled “Some Fundamental Properties ofSpeckle” (J. Opt. Soc. Am. 66, pp. 1145-1149, 1976) and a second paperentitled “Statistical Properties of Laser Speckle Patterns” (Topics inApplied Physics volume 9, edited by J. C. Dainty, pp. 9-75,Springer-Verlag, Berlin Heidelberg, 1984).

There are two types of speckle: objective and subjective speckle. Asnoted in an article by D. Gabor in the IBM Journal of Research andDevelopment, Volume 14, Number 5, Page 509 (1970) “Objective” specklearises from the uneven illumination of an object with a multiplicity ofwaves that interfere at its surface. “Subjective” speckle arises atrough objects even if they are illuminated evenly by a single wave.” Inpractical terms, objective speckle results from scattering in theillumination system while subjective speckle occurs at the projectionscreen. As its name implies objective speckle is not influenced by theviewer's perception of the displayed image. A photographic emulsionspread over the surface of the object would record all of the keycharacteristics of objective speckle. Even a perfect optical systemcannot do better than to reproduce it exactly. Subjective speckle on theother hand arises by a diffraction effect at the receiving optics or,more exactly, by the limitation of the amount of light admitted intoreceiving optics (the eye, in the case of a display). The only remedyfor to subjective speckle is to widen the aperture of the receivingoptics or to perform an equivalent optical process. This is due tofundamental information theory limitations and not any practical opticalconsideration

The characteristics of objective and subjective speckle may beillustrated by considering a typical projection system. The illuminationand beam shaping optics (for example components such as diffusers orfly's eye integrators) generates scattering that eventually creates aspeckle pattern onto the microdisplay panel surface. The projection lensimages this pattern onto the screen giving the objective specklepattern. The screen takes the objective speckle pattern and scatters itinto the viewing space. The human eye only collects a tiny portion ofthis light. Since the objective speckle acts like a coherentillumination field, the diffusion of the screen produces a new specklepattern at the retina with a different speckle grain. This is thesubjective speckle pattern. The subjective speckle pattern will beinfluenced by screen diffuser materials and lenticular structures andother features commonly used in screens. Since a well designedprojection lens usually collects most of the light transmitted throughor reflected by the microdisplay panel, the objective speckle patterngenerated is well reproduced at the screen, allowing for somemodification due to optical aberrations. The cumulative speckle seen bythe eye is the sum of the objective and subjective speckles.

Removing the objective speckle is relatively easy since the specklepattern is well transferred from the illumination to the screen: anychange in the illumination will be transferred to the screen.Traditionally, the simplest way has been to use a rotating diffuser thatprovides multiplicity of speckle patterns while maintaining a uniform atime-averaged intensity profile. This type of approach is often referredto as angle diversity. Note that, if the objective speckle is suppressedat the screen, it will be suppressed at every plane between theprojection lens and the screen.

Suppression of subjective speckle is more difficult. Because of largedisparity between the projection optics and eye optics numericalapertures (or F-numbers), the objective speckle grain is much largerthan the subjective speckle grain. Therefore, the objective speckleprovides a relatively uniform illumination to the screen within oneresolution cell of the eye regardless of the position of the rotatingdiffuser or other speckle reduction means in the illumination path. Forthe purposes of quantifying the subjective speckle it is convenient todefine the speckle contrast as the ratio of the resolution spots of theeye and the projection optic at the screen.

The characteristics of speckle depend on whether it is observed in thenear or far field. The far field of an optical system is the angularspectrum of the plane waves traversing or generated by the opticalsystem. In case of a diffractive optical element such as a ComputerGenerated Hologram (CGH), the far field is a series of points located inthe two dimensional angular spectrum, each point representing theintensity of a specific plane wave diffracted, refracted, reflected ordiffused at a specific angle. If only one beam strikes the opticalelement, no overlap of plane waves occurs, each plane wave beingspatially demultiplexed in the far field. This is not the case for thenear field. The far field effectively at infinity, which according toRayleigh-Sommerfeld theory is any distance after a specific finitedistance, which is a function of the size of the beam (that is, theeffective aperture of the CGH), the wavelength, the size of themicrostructures in the element (amount of beam deflection), and otherfactors. Therefore, in order to change the speckle pattern of anindividual beamlet in the far field, it is best to use phase diversity.Angular diversity would not produce good results, since none of the wavefronts would be overlapping and interfering. However, phase diversitywould create a different phase pattern on a single beamlet and thiswould change the speckle. Speckle patterns in the far field arecharacterized by very small-grained speckle structures.

In the near field (that is any location closer than theRayleigh-Sommerfeld distance), many different wave fronts are interfereand overlap resulting in a very large amount of local wave frontinterference and hence speckle. Therefore, in order to reduce speckle inthe near field, it is advantageous to make slight variations to theangles of the overlapping beamlets. In other words, angular diversitydespeckling schemes will be the most effective. Speckle in the nearfield is characterized by larger grains. The different grain structurein the near and far fields can lead to the erroneous conclusion thatFresnel CGH (near field) give less speckle than Fourier CGHs (farfield). This is not the case; the nature of the speckle is different inthe two cases.

The extent to which speckle can be corrected in the near and far fieldshas implications for the type of despecklers to be used in specificprojector applications. In the case of a laser projector usingtraditional projection imaging apparatus, the image of a microdisplay isnot in the far field of the despeckler, and thus angular diversity wouldbe the most effective solution. In the case of a laser projector usingdiffractive imaging, the image is actually the far field of themicrodisplay itself, and very close to the far field of the despeckler.Therefore, it is best to use a combination of angular diversity andphase diversity.

Techniques for speckle reduction are commonly classified into thecategories of angular, phase and wavelength diversity according to theoptical property used to generate the speckle patterns. Angulardiversity typically relies on the used of rotating diffusers orvibrating screens. Phase diversity is typically provided by electricallycontrolled phase modulators. Wavelength diversity is provided bymultiple laser sources or tuneable single laser sources. In the case oflaser arrays, speckle reduces as the inverse of the square root of thenumber of die. Mechanical methods of suppressing speckle suffer from theproblems of noise, mechanical complexity and size.

It is known that speckle may be reduce by using an electro optic deviceto generate variation in the refractive index profile of material suchthat the phase fronts of light incident on the device are modulated inphase and or amplitude. The published Internal Patent Application No.WO/2007/015141 entitled LASER ILLUMINATOR discloses a despeckler basedon a new type of electro optical device known as an ElectricallySwitchable Bragg Grating (ESBG).

An ESBG in its most basic form is formed by recording a volume phasegrating, or hologram, in a polymer dispersed liquid crystal (PDLC)mixture. Typically, ESBG despeckler devices are fabricated by firstplacing a thin film of a mixture of photopolymerizable monomers andliquid crystal material between parallel glass plates. Techniques formaking and filling glass cells are well known in the liquid crystaldisplay industry. One or both glass plates support electrodes, typicallytransparent indium tin oxide films, for applying an electric fieldacross the PDLC layer. A volume phase grating is then recorded byilluminating the liquid material with two mutually coherent laser beams,which interfere to form the desired grating structure. During therecording process, the monomers polymerize and the HPDLC mixtureundergoes a phase separation, creating regions densely populated byliquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting volumephase grating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thePDLC layer. When an electric field is applied to the hologram viatransparent electrodes, the natural orientation of the LC droplets ischanged causing the refractive index modulation of the fringes to reduceand the hologram diffraction efficiency to drop to very low levels. Notethat the diffraction efficiency of the device can be adjusted, by meansof the applied voltage, over a continuous range from near 100%efficiency with no voltage applied to essentially zero efficiency with asufficiently high voltage applied. U.S. Pat. No. 5,942,157 and U.S. Pat.No. 5,751,452 describe monomer and liquid crystal material combinationssuitable for fabricating ESBG despeckler devices. A publication byButler et al. (“Diffractive properties of highly birefringent volumegratings: investigation”, Journal of the Optical Society of America B,Volume 19 No. 2, February 2002) describes analytical methods useful todesign ESBG despeckler devices and provides numerous references to priorpublications describing the fabrication and application of ESBGdespeckler devices.

The apparatus disclosed in Internal Patent Application No.WO/2007/015141 suffers from the problem that insufficient speckle statesare produced using the ESBG configurations taught therein.

It is a first object of the present invention to provide an ESBGdespeckler device that can overcome the problem of laser speckle.

It is a second object of the present invention to provide a compact,efficient laser display incorporating an ESBG despeckler device that canovercome the problem of laser speckle.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an ESBGdespeckler device that can overcome the problem of laser speckle.

It is a second object of the present invention to provide a compact,efficient laser display incorporating an ESBG despeckler device that canovercome the problem of laser speckle.

In a first embodiment of the invention there is provided an ESBGdespeckler device comprising a least one ESBG element. Said ESBG elementis recorded in a HPDLC sandwiched between transparent substrates towhich transparent conductive coatings have been applied. At least one ofsaid coatings is patterned to provide a two-dimensional array ofindependently switchable ESBG pixels. At least first and second voltagesare applied across each ESBG element. Each ESBG element is characterizedby a first unique speckle state under the first applied voltage and asecond unique speckle state under the second applied voltage. The ESBGdespeckler device is disposed along an illumination optical path.

The ESBG despeckler device is configured to modify the opticalcharacteristics of incoming light to provide a set of speckle cells.Said first and second voltages are points on a time varying voltagecharacteristic wherein the voltage applied to each ESBG pixel iscyclically varied from zero to some specified maximum value at a highfrequency. The effect of the varying voltage is to vary the opticaleffect of the ESBG despeckler device on incoming light in acorresponding fashion. In effect, the ESBG despeckler device generates amultiplicity of different speckle patterns within the human eyeintegration time. A human eye observing the display integrates saidpatterns to provide a substantially de-speckled final image.

In one embodiment of the invention the ESBG despeckler devices areconfigured using two ESBG elements disposed in sequence. The ESBGelements are operated in tandem with alternating voltages applied acrossthe ESBG pixels. The optical effect of each ESBG pixel is varied fromzero to maximum value at a high frequency by applying an electric fieldthat varies in a corresponding varying fashion. Each incremental changein the applied voltage results in a unique speckle phase cell.

In one embodiment of the invention the ESBG despeckler device comprisesidentical first and second ESBG arrays and the waveforms applied tooverlapping elements of said first and second ESBG arrays operate inanti-phase.

In one embodiment of the invention the ESBG despeckler device comprisesidentical first and second ESBG arrays and said second ESBG array isrotated through 180 degrees with respect to said first ESBG array.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element recorded using a Computer Generated Hologram(CGH). The CGH has a first surface and a second surface wherein said CGHis designed to convert a laser beam incident at said first surface intoa multiplicity of beams from said second surface wherein each beam has aunique direction in space and a diffusion angle, wherein said beams havesingle point of origin.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element recorded using a CGH). The CGH has a firstsurface and a second surface wherein said CGH is designed to convert alaser beam incident at said first surface into a multiplicity of beamsfrom said second surface wherein each beam has a unique direction inspace and a diffusion angle, wherein said beams have points of originequally spaced around the periphery of said CGH.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element recorded using a CGH. The CGH comprises anarray of diffracting elements. Each said diffracting elements ischaracterized by a unique light diffusion function.

In one embodiment of the invention the ESBG despeckler device comprisesidentical first and second ESBG arrays each containing selectivelyswitchable ESBG pixels.

In one embodiment of the invention the ESBG despeckler device comprisesidentical first and second ESBG arrays containing selectively switchableESBG pixels. Each ESBG pixel is characterized by a unique gratingvector.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels each said ESBG pixel converts incident collimatedlight into divergent light.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels each ESBG pixel converts incident light intodiffuse light.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels and at least one of the ESBG arrays provides aset of Hadamard diffusers.

In one embodiment of the invention the ESBG despeckler device stack ofsimilarly configured ESBG arrays.

In one embodiment of the invention the ESBG despeckler device comprisesa stack of ESBG arrays designed to operate on red, green or blue light.

In one embodiment of the invention the ESBG despeckler device comprisesESBG arrays disposed adjacent to each other in a plane.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels the ESBG pixels substantially overlap in theillumination beam cross section.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels the ESBG pixels are offset by a fraction of theESBG element width in at least one of the vertical or horizontal arrayaxes in the illumination beam cross section.

In one embodiment of the invention in which the ESBG despeckler devicecomprises identical first and second ESBG arrays containing selectivelyswitchable ESBG pixels the ESBG pixels are offset by a integer number ofESBG element width in at least one of the vertical or horizontal arrayaxes in the illumination beam cross section.

In one embodiment of the invention the ESBG despeckler device furthercomprises a diffractive optical element for converting incident off axislight into a direction normal to the surfaces of the ESBG despecklerdevice.

In one embodiment of the invention the ESBG despeckler device furthercomprises a diffractive illumination profile shaping element.

In one embodiment of the invention the ESBG despeckler device furthercomprises an electrically controllable phase modulator operative toprovide phase retardation.

In one embodiment of the invention the ESBG despeckler further comprisesan electro-optical polarisation switch providing a phase shift of 0 or πradians. The polarization switch may be a sub wavelength grating. Thepolarization switch is randomly switched with respect to the pixelateddiffuser.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element wherein said ESBG element has a first phaseretarding characteristic under a first voltage and a second phaseretarding characteristic under a second voltage.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element wherein said ESBG element has a first lightdiffusing characteristic under a first voltage and a second lightdiffusing characteristic under a second voltage.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element which encodes the optical characteristics ofan axicon.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element which encodes the optical characteristics of asub wavelength grating phase retarder.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element which encodes the optical characteristics of adiffuser.

In one embodiment of the invention the ESBG despeckler device comprisesa stack of three ESBG elements each having substantially the sameoptical function but designed to operate on red, green and blue lightrespectively.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element configured as either a variable diffuser avariable subwavelength grating or a variable axicon.

In one embodiment of the invention the ESBG despeckler device comprisesred, green and blue ESBG elements disposed adjacent to each other.

In one embodiment of the invention there is provided a despecklercomprising a first ESBG array a second ESBG array and a DOE. The ESBGarrays are operated in anti-phase. The ESBG arrays and the DOE arealigned with their surface orthogonal to an optical axis. The DOEdirects on-axis incident laser light into an off-axis direction. Thefirst and second ESBG arrays each deflect incident off-axis light intoan on-axis direction. Said DOE may be a holographic element such as aBragg hologram, Said DOE may be a ESBG

In one embodiment of the invention there is provided a despecklercomprising a first ESBG array a second ESBG array and a DiffractiveOptical Element (DOE). The ESBG arrays and the DOE are aligned withtheir surface orthogonal to an optical axis. The DOE directs off axisincident laser light into a direction parallel to said optical axis. Thefirst ESBG device deflects incident on-axis light into an off axisdirection. The second ESBG device deflects light incident t at saidoff-axis direction light into an on-axis direction.

In one embodiment of the invention there is provided a despecklercomprising a first ESBG array a second ESBG array and a DOE. The ESBGarrays and the DOE are aligned with their surface orthogonal to anoptical axis. The DOE directs on-axis incident laser light into anoff-axis direction. The first and second ESBG devices are each operativeto deflect incident off-axis light into an on-axis direction.

In one embodiment of the invention there is provided a ESBG despecklerdevice comprising an array in which the ESBG pixels encode diffusioncharacteristics.

In one embodiment of the invention there is provided a ESBG despecklerdevice comprising an array in which the ESBG pixels encode keystonecorrection.

In one embodiment of the invention the ESBG despeckler device comprisesat least one ESBG element recorded by means of an apparatus comprise alaser source, a beam expanding lens system, a beam splitter, a mirror, asecond lens, a computer generated hologram (CGH) and a cell containingthe HPDLC mixture into which the ESBG is recorded. The CGH is designedto generate a set of beamlets from a single input beam.

In one embodiment of the invention there is provided ESBG despecklerdevice comprising two ESBG arrays configured to provide switchableoptical path differences wherein the ESBG pixels substantially overlap.The pixels of the first ESBG array deflect normally incident collimatedlight through a specified angle. The pixels of the second ESBG arraydiffract incident light at said angle into direction normal to thesecond ESBG array. When the ESBG pixels are not in their diffractingstates incident light is transmitted without substantial deviation. Thelateral displacement of the beam when the ESBG pixels are in adiffracting state results in an optical path difference given by theproduct of the separation of the ESBG arrays, the average refractiveindex of the optical path between the arrays and the tangent of thediffraction angle.

In one embodiment of the invention there is provided an ESBG despecklerdevice in comprising three ESBG arrays configured to provide switchableoptical path differences. The apparatus comprises three ESBG arraysaligned in series along an optical axis. The pixels of the first ESBGarray deflects normally incident collimated light through a first angle.The pixels of second ESBG diffracts incident light at said first angleinto a direction parallel to the axis. The pixels of the third ESBGarray diffract light incident at said first angle such into a directionparallel to the optical axis. When the ESBG arrays pixels are not intheir diffracting states incident light is transmitted withoutsubstantial deviation. When the second and third ESBG array pixels arenot in their diffracting states the diffracted light is transmittedwithout deviation. The lateral displacement of the incident light whenthe ESBG array pixels are in a diffracting state results in an opticalpath difference given by the product of the separation of first andsecond ESBG arrays or second and third ESBG arrays, the averagerefractive index of the optical path between said gratings and thetangent of the diffraction angle.

In one embodiment of the invention there is provided a method offabricating an ESBG array for use in the invention comprising thefollowing steps:

a first step in which a substrate to which a transparent electrode layerhas been applied is provided;

a second step in which portions of said transparent electrode layer areremoved to provide a patterned electrode layer including at least oneESBG pixel pad;

a third step in which a layer of UV absorbing dielectric material isdeposited over said patterned electrode layer;

a fourth step in which the portion of said UV absorbing dielectricmaterial overlapping said ESBG pixel pad is removed;

a fifth step in which a second substrate to which a transparentelectrode layer has been applied is provided;

a sixth step in which the transparent electrode layer of the secondsubstrate layer is etched to provide a patterned electrode layerincluding a second ESBG pixel pad substantially identical to andspatially corresponding with the first ESBG pixel pad;

a seventh step in which the two substrates processed according to theabove steps are combined to form a cell with the electrode coatedsurfaces of the two cells aligned in opposing directions and having asmall separation;

an eight step in which the cell is filled with a PDLC mixture;

a ninth step in which the cell face formed by the first substrate isilluminated by crossed UV laser beams, and simultaneously the cell faceformed by the second is illuminated by an incoherent UV source formingan HPDLC region confined to the region between the first and second ESBGpixels and surrounded by a PDLC region

In one embodiment of the invention there is provided a laser displaycomprising at least one laser die, a flat panel display, a projectionlens and an ESBG despeckler device disposed along the path of the beamemitted by said laser. A variable voltage generator is coupled to theESBG despeckler device.

In one embodiment of the invention there is provided a laser displayaccording to the principles of the invention comprises a multiplicity oflaser emitter die configured as a two dimensional array, a flat paneldisplay, a projection lens and an ESBG despeckler device disposed alongthe path of the beam emitted by said laser. A variable voltage generatoris coupled to the ESBG despeckler device.

In one embodiment of the invention a laser display further comprises anoptical element disposed along the laser beam paths for shaping theintensity profile and cross sectional geometry of the illuminator beam.

In one embodiment of the invention a laser display further comprises alight integrator pipe may be disposed in the light path after the ESBGdespeckler device.

In one embodiment of the invention a laser display further comprises amicro lens element may be disposed between the laser die and the ESBGdespeckler device.

In an alternative embodiment of the invention the ESBG despeckler deviceis disposed between the flat panel display and the projection lens.

In a further embodiment of the invention the ESBG despeckler device isdisposed within a projection lens.

In one embodiment of the invention a laser display further comprises adiffractive beam steering element disposed between the laser source andthe ESBG despeckler device.

In one embodiment of the invention directed at providing coloursequential red green and blue laser illumination there are providedseparated red, green and blue laser modules each comprising at least onelaser source, beam expansion and collimation lens system and an ESBGdespeckler device further comprising a first ESBG array and a secondESBG array. The red green and blue beams are reflected into a commondirection by means of dichroic filter. The reflected beams directedtowards a display panel. A projection lens projects an image of thedisplay panel onto a screen.

In a further embodiment of the invention there is provided an edgeilluminator comprising an ESBG despeckler device wherein the substratesof the ESBG despeckler device provide a total internal reflection (TIR)light guiding structure. An input light-coupling optical elementprovides a means for injecting light from a laser source into the lightguiding structure. An output light-coupling optical element provides ameans for ejecting light from the light guide into an illumination pathdirected at a flat panel display.

In a further embodiment of the invention there is provided an edgeilluminator comprising an ESBG despeckler device. The ESBG despecklerdevice comprises two or more ESBG layers. The substrates of the ESBGlayers together form a TIR light guiding structure.

In a further embodiment of the invention there is provided a scrollingedge illuminator comprising an ESBG despeckler device wherein thesubstrates of the ESBG despeckler device provide a TIR light guidingstructure. The ESBG despeckler device comprises at least one ESBGelement. At least one ESBG element has electrodes are divided into anumber of parallel stripes that define a corresponding numberindependently controllable ESBG stripes.

In a further embodiment of the invention there is provide an edge litESBG despeckler device that also performs the function of a spatiallight modulator.

In a further embodiment of the invention there is provided an edgeilluminator comprising an ESBG despeckler device wherein the substratesof the ESBG despeckler device provide a TIR light guiding structure andfurther comprising a second trapezoidal light guiding structure disposedadjacent the ESBG substrates. The ESBG despeckler device contains ESBGportions for coupling light into and out of said second light guidingstructure

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings wherein like index numerals indicate like parts.For purposes of clarity details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 2 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 3A is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 3B is a chart showing a first ESBG applied voltage characteristic.

FIG. 3C is a chart showing a second ESBG applied voltage characteristic.

FIG. 4 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 5A is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 5B is a front elevation of a first detail of the embodiment of FIG.5A.

FIG. 5C is a front elevation of a first detail of the embodiment of FIG.5A.

FIG. 6 is a schematic side elevation view of a further embodiment of theinvention.

FIG. 7A is a schematic side elevation view of a prior art device relatedto one particular embodiment of the invention.

FIG. 7B is a schematic side elevation view of one particular embodimentof the invention.

FIG. 7C is a schematic side elevation view of a prior art device relatedto one particular embodiment of the invention.

FIG. 8 is a schematic side elevation view of a one embodiment of theinvention.

FIG. 9 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 10 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 11 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 12 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 13 is a schematic side elevation view of a projection lensaccording to embodiment of the invention.

FIG. 14 is a schematic front elevation view of an ESBG array used in oneembodiment of the invention.

FIG. 15 is a schematic front elevation view of an ESBG array used in oneembodiment of the invention.

FIG. 16 is a schematic front elevation view of an ESBG array used in oneembodiment of the invention.

FIG. 17 is a schematic front elevation view of an ESBG array used in oneembodiment of the invention.

FIG. 18 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 19 is a schematic side elevation view of a one particularoperational embodiment of the invention.

FIG. 20 is a schematic side elevation view of a one particularoperational embodiment of the invention.

FIG. 21 is a series of schematic front elevation views of an ESBGelement at successive stages in its fabrication according to the basicprinciples of invention.

FIG. 22 shows a series of schematic side elevation views of an ESBGelement at successive stages in its fabrication according to the basicprinciples of the invention.

FIG. 23 is a side elevation view of the assembled ESBG element.

FIG. 24 is a side elevation view of the assembled ESBG showing therecording process.

FIG. 25 is a flow diagram of a method of fabricating the ESBG accordingto the principles of the invention.

FIG. 26 is a plan view of a particular embodiment of an ESBG electrodearray used in an embodiment of the invention.

FIG. 27 is a plan view of a particular embodiment of an ESBG electrodearray used in an embodiment of the invention.

FIG. 28 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 29 is a schematic side elevation view of a one particularoperational embodiment of the invention.

FIG. 30 is a schematic side elevation view illustrating one method ofrecording an ESBG array for use with the invention.

FIG. 31 is a schematic side elevation view illustrating one method ofrecording an ESBG array for use with the invention.

FIG. 32A is a schematic side elevation view illustrating one method ofrecording an ESBG array for use with the invention.

FIG. 32B is a front elevation view showing one aspect of a computergenerated hologram used to record an ESBG array for use with theinvention.

FIG. 33A is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 33B is a schematic side elevation view of one particularoperational embodiment of the invention.

FIG. 34 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 35A is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 35B is a schematic side elevation view of one particularoperational embodiment of the invention.

FIG. 35C is a chart showing a first ESBG applied voltage characteristicused in the embodiment of FIG. 35D is a chart showing a second ESBGapplied voltage characteristic used in the embodiment of FIG. 35D.

FIG. 36 is a schematic side elevation view of a laser display accordingto one embodiment of the invention.

FIG. 38 is a schematic side elevation view of one particular embodimentof the invention.

FIG. 39 is a schematic side elevation view of another particularembodiment of the invention.

FIG. 40 is a schematic front elevation view of aspects of a computergenerated hologram used to record an ESBG array used in embodiments ofthe invention.

FIG. 41 is a schematic front elevation view of a computer generatedhologram used to record an ESBG array used in embodiments of theinvention.

FIG. 42 is a three dimensional schematic illustration of a ESBG arrayswitching scheme used in the invention.

FIG. 44 is a table showing the sequence of logic states applied to therows of an ESBG array used in the invention.

FIG. 45 is a table showing the sequence of logic states applied to thecolumns of an ESBG array used in the invention.

FIG. 46 is a schematic front elevation view of the 3×3 ESBG array moduleused in one embodiment of the invention.

FIG. 47 is a chart illustrating the waveform applied to the ESBG in oneembodiment of the invention.

FIG. 48 is a table illustrating a typical speckle sample generatingprocess used in the embodiment of FIG. 46.

FIG. 49 is a schematic illustration of the complete set ESBG arraypatterns generated using the embodiment of FIG. 46.

FIG. 50 is a schematic side elevation view of a an ESBG despecklerdevice using ESBG arrays operating in random anti phase and furthercomprising a polarization switch stage.

FIG. 51 is a schematic side elevation view of one embodiment of theinvention providing an edge lit ESBG despeckler device.

FIG. 52 is a schematic side elevation view of one embodiment of theinvention providing an edge lit ESBG despeckler device.

FIG. 53 is a schematic side elevation view of one embodiment of theinvention providing an edge lit ESBG despeckler device.

FIG. 54 is a schematic side elevation view of one embodiment of theinvention providing an edge lit ESBG despeckler device.

FIG. 55 is a schematic side elevation view of one embodiment of theinvention providing a colour edge lit scrolling illuminator.

FIG. 56 is a schematic front elevation view of an aspect of a scrollingilluminator according to the principles of the invention.

FIG. 57 is a schematic front elevation view of an aspect of a scrollingilluminator according to the principles of the invention.

FIG. 58 is a schematic front elevation view of an aspect of a scrollingilluminator according to the principles of the invention.

FIG. 59 is a schematic side elevation view of one embodiment of theinvention providing an edge lit scrolling ESBG despeckler device.

FIG. 60 is a schematic side elevation view of one embodiment of theinvention providing an edge lit scrolling ESBG despeckler device.

FIG. 61 is a schematic side elevation view of one embodiment of theinvention providing an edge lit scrolling ESBG despeckler device.

FIG. 62 is a schematic side elevation view of one embodiment of theinvention providing an edge lit scrolling ESBG despeckler device.

FIG. 63 is a schematic side elevation view of one embodiment of theinvention providing a colour edge lit scrolling ESBG despeckler device.

FIG. 64A is a schematic plan view of an embodiment of the invention thatuses path switchable light guiding structure.

FIG. 64B is a schematic side elevation view of an embodiment of theinvention that uses path switchable light guiding structure.

FIG. 65A is a schematic plan view of another embodiment of the inventionthat uses path switchable light guiding structure.

FIG. 65B is a schematic side elevation view of another embodiment of theinvention that uses path switchable light guiding structure.

FIG. 65C is a schematic front elevation view of an aspect of anotherembodiment of the invention that uses path switchable light guidingstructure.

DETAILED DESCRIPTION OF THE INVENTION

It is a first object of the present invention to provide an ESBGdespeckler device that can overcome the problem of laser speckle.

It is a second object of the present invention to provide a compact,efficient laser display incorporating an ESBG despeckler device that canovercome the problem of laser speckle.

To assist in clarifying the basic principles of the despeckler devicethe invention will be described in relation to a practical laser displaywhich comprises a laser source comprising one or more red, green or bluelaser die, a flat panel microdisplay and projection optics. It will beclear that the despeckler embodiment to be described is not restrictedto application in laser display configurations of the type described.

For the purposes of explaining the invention an ESBG despeckler devicewill be understood to comprise one or more ESBGs layers or cells eachcomprising an ESBG encapsulated between parallel transparent glass wallsaccording to the principles to be discussed below. In some cases an ESBGlayer or cell may simply be referred to as an ESBG. An ESBG array willrefer to an ESBG with switching electrodes patterned such thatindividual ESBG pixels can be switched selectively.

It will be apparent to those skilled in the art that the presentinvention may be practiced with only some or all aspects of the presentinvention as disclosed in the following description. For the purposes ofexplaining the invention well-known features of laser technology andlaser displays have been omitted or simplified in order not to obscurethe basic principles of the invention.

Parts of the following description will be presented using terminologycommonly employed by those skilled in the art of optics and laserdisplays in particular.

It should also be noted that in the following description of theinvention repeated usage of the phrase “in one embodiment” does notnecessarily refer to the same embodiment.

FIG. 1A shows a schematic side elevation view of one embodiment of theinvention in which a laser display comprises a laser source 1 and anElectrically Switchable Bragg Grating (ESBG) device 2, which is disposedalong the laser beam path and a projection optical system generallyindicated by 4. The laser source 1 comprises at least a single laseremitter die providing monochromatic light. The ESBG drive electronicsare indicated by 3. The laser and ESBG despeckler device form part of anapparatus for illuminating an electronic display to provide a viewableimage. The projection optical system may comprise an electronic displaypanel such as an LCD, a projection lens, and relay optics for couplingthe ESBG despeckler device to the display panel, filters, prisms,polarizers and other optical elements commonly used in displays. Thefinal image is projected onto a projection screen 5. A lens 7 may beused to convert diverging laser emission light 1100 into a collimatedbeam 1200. The collimated beam is diffracted into a direction 1300 bythe ESBG despeckler device. The optical system 4 forms a diverging beam1400, which illuminates the screen 5. The details of the projectionoptical system do not form part of the invention. The invention is notrestricted to any particular type of display configuration. At least oneviewable surface illuminated by the laser light exhibits laser speckle.Said viewable surface may be at least one of the projection screen 5 oran internal optical surface within the projection optical system.Although a rear projection screen is illustrated in FIG. 1A theinvention may also be used in front projection.

The invention is not restricted to the projection of informationdisplayed on an electronic display panel. The invention may also beapplied to reducing speckle in laser scanner displays in which theprojection optical system would typically comprised beam scanningcomponents and light modulators well know to those skilled in the art ofscanned laser displays. Although in FIG. 1A the ESBG despeckler deviceis disposed between the lens 7 and the projection optical system 4, theinvention does not assume any particular location for the ESBG.Advantageously, the ESBG despeckler device is located in a collimatedbeam path to provide high diffraction efficiency.

An ESBG despeckler device according to the principles of the inventiontypically comprises at least one ESBG element. Each ESBG layer has adiffracting state and a non-diffracting state. Typically, the ESBGelement is configured with its cell walls perpendicular to an opticalaxis. An ESBG element diffracts incident off-axis light in a directionsubstantially parallel to the optical axis when in said active state.However, each ESBG element is substantially transparent to said lightwhen in said inactive state. An ESBG element can be designed to diffractat least one wavelength of red, green or blue light. In the embodimentsto be discussed in the following description of the invention at leastone ESBG layer in the ESBG despeckler device is configured as an arrayof selectively switchable ESBG pixels.

ESBG despeckler devices for reducing speckle according to the principlesof the present invention are configured to generate set of uniquespeckle patterns within an eye resolution cell by operating on theangular and/or phase characteristic of rays propagating through the ESBGdespeckler device. The ESBG despeckler devices disclosed herein may beused to overcome both objective and subjective speckle.

As will be explained below, in any of the embodiments of the inventionthe ESBG despeckler device may comprise more than one ESBG layerfabricated according to the principles described above. Furthermore, theESBG despeckler device may be configured in several different ways tooperate on one or more of the phase, and ray angular characteristics ofincoming light.

Varying the electric field applied across the ESBG despeckler devicevaries the optical effect of the ESBG despeckler device by changing therefractive index modulation of the grating. Said optical effect could bea change in phase or a change in beam intensity or a combination ofboth. The optical effect of the ESBG despeckler device is varied fromzero to a predetermined maximum value at a high frequency by applying anelectric field that varies in a corresponding varying fashion. Saidvariation may follow sinusoidal, triangular, rectangular or other typesof regular waveforms. Alternatively, the waveform may have randomcharacteristics. Each incremental change in the applied voltage resultsin a unique speckle phase cell. A human eye 5 observing the display ofFIG. 1A integrates speckle patterns such as those illustrated by 500 a,500 b to provide a substantially de-speckled final image.

The basic principles of speckle reduction using angular diversity areillustrated schematically in FIG. 1B. The projection beam axis and theeye line of sight are assumed to lie on a common optical axis indicatedby 1201. The exit pupil of the projection systems is indicated by 1202and the entrance pupil of the eye is indicated by 1203. The diameters ofthe projection and eye pupils are D₁ and D₂ respectively and theprojection and eye pupils are located at distances of R₁ and R₂respectively from a transmissive screen 5. The projection lightindicated by 1204 is provided by an optical system such as the oneillustrated in FIG. 1A. The light detected by eye indicated by 1203 isimaged onto the retina. In order for the eye to detect the optimumspeckle reduction the eye must resolve the laser illuminated area intoresolution spots having a resolution spot size indicated by 1501 whichis greater than or approximately equal to a speckle surface resolutioncell such as the one indicated by 1502 For light of wavelength λ thediameter of the eye resolution spot is given by the Airy point spreadfunction diameter 2.44λ R₁/D₁. The diameter of the speckle resolutioncell such as 1501 is given by 2.44λ R₂/D₂. Temporally varying the phasepattern faster than the eye temporal resolution destroys the lightspatial coherence, thereby reducing the speckle contrast.

The invention does not assume any particular type of laser or laserconfiguration. The laser source may be a single die or an array of die.In one embodiment of the invention shown in the schematic side elevationview of FIG. 2, the laser source comprises a multiplicity of laseremitter die configured as a two-dimensional array 10. A microlens array70 containing microlens elements may be provided. For example in thearray shown in FIG. 2 the lens element 71 converts diverging light 1101from laser element 11 into a collimated beam 1201. The microlens arraydoes not form part of the invention. The ESBG despeckler device 2comprises at least one ESBG array where each array contains amultiplicity of separately controllable ESBG elements similar to the oneshown in FIG. 1. As shown in FIG. 2, a collimated beam 1201 propagatesthrough an ESBG array element 21. Each ESBG element is operative toreceive light from one laser die. In one operational embodiment of theinvention said lasers and said ESBGs are operated such that theillumination from the lasers is provided in a time sequence.

In a further embodiment of the invention, which is also illustrated byFIG. 2, the lasers emit light simultaneously. Each ESBG despecklerdevice provides a unique set of speckle phase cells from itscorresponding laser die.

In further embodiments of the invention the ESBG despeckler devices inany of the embodiments described above may be configured using multipleESBG elements disposed in sequence. For example, referring to the sideelevation view of FIG. 3A, it will be seen that the ESBG element of FIG.1A has been replaced by the two ESBG elements 2A, 2B, which arecontrolled by the ESBG controller 30. The ESBG elements are operated intandem with alternating voltages applied across the ESBG layers. Theoptical effect of each ESBG despeckler device is varied from zero tomaximum value at a high frequency by applying an electric field thatvaries in a corresponding varying fashion. Each incremental change inthe applied voltage results in a unique speckle phase cell. Referring toFIG. 3B which is a chart showing voltage versus time applied to the ESBGelements it will be seen that there is a phase lag between the voltages1001,1002 applied across the ESBGs. The effect of applying suchwaveforms is that the average intensity 1003 of the speckle phase cellsremains substantially constant, thereby satisfying the statisticalrequirements for speckle reduction. Other types of waveforms may beapplied, for example sinusoidal, triangular, rectangular or other typesof regular waveforms. Alternatively, it may be advantageous instatistical terms to use waveforms based on a random stochastic processsuch as the waveforms illustrated in the chart of FIG. 3C. The chart ofFIG. 3C shows voltage versus time characteristics for phase shiftedrandom voltages 2001,2002 applied to the ESBG elements. Again, theeffect of applying the waveforms is that the average intensity 2003 ofthe speckle phase cells remains substantially constant.

In any of the embodiments of the invention beam-shaping element disposedalong the laser beam paths may be used to shape the intensity profile ofthe illuminator beam. Laser array tend to have emitting surface aspectratios of that are incompatible with the aspect ratios of commonmicrodisplay devices. FIG. 4 shows a side elevation view of anilluminator similar to the embodiment of FIG. 1, which further comprisesthe beam-shaping element 8. The beam-shaping element may be a lightshaping diffuser such as the devices manufactured by POC Inc. (USA) or aComputer Generated Hologram. Other technologies may be used to providethe light shaping function.

The ESBG despeckler device may be configured to perform the additionalfunction of beam steering. This may be advantageous with laser arrays inwhich the die has large separations. In such a configuration at leastone ESBG layer is configured to generate speckle phase cells while afurther one or more ESBG layers are configured to diffract incidentlight into a specified direction. Desirably, the second ESBG operatesaccording to the basic principles described in U.S. Pat. No. 6,115,152.entitled HOLOGRAPHIC ILLUMINATION SYSTEM.

In one embodiment of the invention shown in the schematic side elevationview of FIG. 5A the laser display comprises a multiplicity of laseremitter die configured as a two dimensional array 10, a flat paneldisplay, a projection lens and an ESBG despeckler device comprising afirst array of separately controllable ESBG elements 2A and a secondarray of separately controllable ESBG elements 2B. The illuminatorfurther comprises a multiplicity of ESBG elements configured as a stack40. The first ESBG array operates in a similar fashion to the ESBGdespeckler device illustrated in FIG. 2. However the function of thesecond ESBG array is to deflect beams from the laser die towards theESBG stack 40. The illuminator may further comprise the microlens array70. The ESBG stack directs light beams from said laser die towards theviewer. For example the converging light from the die 11 is collimatedby the microlens element 71 into the beam direction 1201. The angular orpolarization characteristics of the beam are modified by the ESBGelement 21A. The ESBG element 21B deflects the beam 1201 into the beamdirection 1300. The beam 1300 is deflected into the direction 1400 byelement 41 of the ESBG stack 40. FIG. 5B is a front elevation view of aportion of the microlens array. FIG. 5C is a front elevation view of aportion of the laser die array. The configuration of FIG. 5A may be usedin conjunction with any of the speckle reduction methods disclosed inthe present application. It will be clear that that by eliminating thefirst ESBG array from the apparatus shown in FIG. 5 there is provide ameans for combining beams from multiple laser sources into a commondirection. In a further embodiment of the invention the functionsperformed by the ESBG arrays in FIG. 5 may be combined in a single ESBGlayer.

As indicated above ESBG despeckler devices according to the principlesof the present invention can be configured to provide a range ofspatio-temporal speckle averaging schemes. In any of the embodimentsshown in FIGS. 1-5 the ESBG despeckler device could be configured as avariable subwavelength grating. Essentially the ESBG despeckler deviceacts as a variable phase retarder. FIG. 6 shows a cross section view ofa sub wavelength grating 50. The light regions 51 represent polymerfringes. The shaded regions 52 represent PDLC fringes. The grating pitchmust be much larger than the incidence light wavelength. Light 600incident at an angle θ continues to propagate at the same angle afterpassing through the grating 601. Sub-wavelength gratings are highspatial frequency gratings such that only the zero order 600, forwarddiffracted 601 and backward “diffracted” waves 602 propagate. All higherdiffracted orders are evanescent. Incident light waves cannot resolvethe sub-wavelength structures and see only the spatial average of thegrating material properties.

An ESBG configured as a sub wavelength grating exhibits a property knownas form birefringence whereby polarized light that is transmittedthrough the grating will have its polarization modified. Subwavelengthgratings behave like a negative uniaxial crystal, with an optic axisperpendicular to the PDLC planes. The basic principles of sub wavelengthgratings discussed is Born and Wolf, Principles of Optics, 5th Ed., NewYork (1975). It is known that the retardance is related to the netbirefringence, which is the difference between the ordinary index ofrefraction and the extraordinary index of refraction of thesub-wavelength grating.

Where the combined thickness of the PDLC plane and the polymer plane issubstantially less than an optical wavelength the grating will exhibitform birefringence. The magnitude of the shift in polarization isproportional to the length of the grating. By carefully selecting thelength of the subwavelength grating for a given wavelength of light, onecan rotate the plane of polarization. Thus, the birefringence of thematerial may be controlled by simple design parameters and optimized toa particular wavelength, rather than relying on the given birefringenceof any material at that wavelength.

It is known that the effective refractive index of the liquid crystal isa function of the applied electric field, having a maximum when thefield is zero and a value equal to that of the polymer at some value ofthe electric field. Thus, by application of an electric field, therefractive index of the liquid crystal and, hence, the refractive indexof the PDLC plane can be altered. When the refractive index of the PDLCplane exactly matches to the refractive index of the polymer plane, thebirefringence of the subwavelength grating can be switched off. To forma half-wave plate, the retardance of the subwavelength grating must beequal to one-half of a wavelength and to form a quarter-wave plate, theretardance must be equal to one-quarter of a wavelength.

ESBGs based on sub-wavelength gratings as described above may beoperated in tandem with alternating voltages applied across the ESBGlayers according to the principles illustrated in FIG. 3. The retardanceof each ESBG is varied from zero to maximum value at a high frequency byapplying an electric field that varies in a corresponding varyingfashion. Each incremental change in the applied voltage results in aunique speckle phase cell. The effect of applying waveforms such asthose illustrated in FIG. 3 is that the average intensity of the specklephase cells remains substantially constant thereby satisfying thestatistical requirements for speckle reduction. Despeckling schemesbased on controlling retardance are sometimes referred to aspolarization diversity schemes.

ESBG despeckler devices according to the principles of the presentinvention can be also configured as variable axicon devices. In suchembodiments of the invention the ESBG acts as a variable phase retarder.According to the basic theory of axicons, a uniform plane wave passingthrough an infinite axicon has a transverse intensity profilerepresented by a first order Bessel function. The intensity profile isconstant along the path giving what is effectively a non-diffractingbeam. The basic principles of axicons are discussed in an article by J.H. McLeod entitled “Axicons and Their Uses” (JOS A, 50 (2), 1960, p.166) and another article by R. M. Herman et T. A. Wiggins entitled“Production and uses of diffraction less beams” (JOSA A, 8 (6), 1991).Practical axicons use collimated Gaussian input beams and generateoutput beams that are referred to in the literature as a Bessel-Gaussbeams. Classical axicons are typically conical single element lenses.The transverse intensity distribution at a specific position is createdby constructive interference from a small annulus of rays incident onthe axicon. Beam intensity is characterized by a intense central regionencircled by rings of lower intensity. Each ring contain same amount ofenergy. Axicons have minimal optical power imparting only a smalldeviation to the incoming beam.

ESBG despeckler devices based on axicons as described above may operatedin tandem with alternating voltages applied across the ESBG layersaccording to the principles illustrated in FIG. 3. The retardance ofeach ESBG axicon device is varied from zero to a predetermined maximumvalue at a high frequency by applying an electric field that varies in acorresponding varying fashion. Each incremental change in the appliedvoltage results in a unique speckle phase cell. The effect of applyingwaveforms such as those illustrated in FIG. 3 is that the averageintensity of the speckle phase cells remains substantially constantthereby satisfying the statistical requirements for speckle reduction.In one embodiment of the invention ESBG despeckler devices based onaxicons could be configured in tandem. In such a configuration laserwavefronts will not be diffracted but will only experience phaseretardation. The diffracted beams substantially overlay thenon-diffracted beams. Both diffracted and non-diffracted beams undergophase retardation.

FIG. 7A is a schematic side elevation view of one configuration 60 of apair of conical lens axicons 61,62. FIG. 7B is a schematic sideelevation view of an ESBG despeckler device 70 comprising a pair of ESBGlayers 71,72 having optical characteristics equivalent to conical lensaxicons 61,62 respectively. FIG. 7C is a schematic side elevation viewof an alternative arrangement of conical lens axicons 80 comprising apair of conical lens axicons 81,82 which could be encoded into the ESBGlayers 71,72 respectively.

ESBG despeckler devices according to the principles of the presentinvention can be also configured as variable diffusers or scatterers. Avariable diffuser is provided by recording diffusing characteristics into an ESBG layer using procedures well known to those skilled in the artof holography. Conventionally, holographic optical element withdiffusing characteristics are recorded by using a holographic cross beamrecording apparatus with a diffuser inserted into one of the recodingbeams. ESBGs characterized as diffusers may be operated in tandem withalternating voltages applied across the ESBG layers according to theprinciples illustrated in FIG. 3. FIG. 8 is a schematic side elevationview of one configuration 90 of an ESBG despeckler device comprising apair of ESBG diffusers 90A,90B. The transmittance of each ESBG is variedfrom zero to a predetermined maximum value at a high frequency byapplying an electric field that varies in a corresponding varyingfashion. Each incremental change in the applied voltage results in aunique speckle phase cell. The effect of applying waveforms such asthose illustrated in FIG. 3 is that the average intensity of the specklephase cells remains substantially constant thereby satisfying thestatistical requirements for speckle reduction. Despeckling schemesbased on diffusers or scatters are sometimes referred to as angulardiversity schemes.

In a further embodiment of the invention shown in FIG. 9 which issimilar to that illustrated in FIG. 2 it will be seen that the ESBGarray 2 of FIG. 2 has been replaced by the two ESBGs 2 a and 2 b, whichare controlled by the ESBG controller 30. The ESBGs 2 a,2 b may encodeaxicons, sub-wavelength gratings or diffusers. As discussed in thepreceding paragraphs, the ESBGs 2 a and 2 b are operated in tandem withalternating voltages applied across the ESBG layers. The angular orpolarization effect of each ESBG array cell is varied at a highfrequency by applying an electric field that varies in a correspondingvarying fashion. Each incremental change in the applied voltage resultsin a unique speckle phase cell. The laser source comprises amultiplicity of laser emitter die configured as a two-dimensional array10. A microlens array 70 containing elements may be provided. Forexample in the array shown in the Figure the lens element 71 convertsdiverging light 101 from laser element 11 into a collimated beam 201.The beam 201 propagates through the ESBG array elements 21 a and 21 b insequence.

A laser display according to the principles of the invention is shown inthe side elevation view of FIG. 10. The laser display comprises a lasersource 1 and an Electrically Switchable Bragg Grating (ESBG) device 2,which is disposed along the laser beam path. The apparatus of FIG. 10further comprises a beam expander 73 a lens system indicated by 74 and aprojection lens indicated by 75. There is further provided a flat paneldisplay 45. The beam expander converts the laser output beam indicatedby 1701 into the expanded beam indicated by 1702. The beam emerging fromthe ESBG despeckler device is indicated by 1703 a. The ESBG despecklerdevice, which is not shown in detail, may be based on any of the ESBGdespeckler devices discussed above. The ESBG despeckler device may be anarray of selectively controllable cells as discussed above. In certaincases the ESBG despeckler device may comprise a single cell. The ESBGdespeckler device may comprise a stack of similarly configured ESBGarrays or single cells. The ESBG despeckler device may include ESBGarrays designed to operate on red, green or blue light. The ESBGdespeckler device may comprise arrays disposed adjacent to each other ina plane. In FIG. 10 the beam 1703 a corresponds to the light emittedfrom a single cell of the ESBG despeckler device. The lens system 74transforms the beam 1703 a into the beam 1703 b forming an illuminationpath that covers the active area of the flat panel display 45. Theprojection lens collects the image light indicated by 1705 from the flatpanel display and focuses light indicated by 1705 to form an image atthe screen 5. The laser source 1 comprises at least a single laseremitter die. Typically, the laser source comprises separate red, greenand blue die. Alternatively, each of the red, green and blue lights maybe provided by arrays of die. The invention is not restricted to anyparticular laser source configuration. The ESBG drive electronics arenot illustrated. The flat panel display may be an LCD or any other typeof device commonly used in video projection. The apparatus may furthercomprise relay optics, beam folding mirrors, light integrators, filters,prisms, polarizers and other optical elements commonly used in displays

Another laser display according to the principles of the invention isshown in the side elevation view of FIG. 11. The laser display comprisesa laser source 1 and an Electrically Switchable Bragg Grating (ESBG)device 2, which is disposed along the laser beam path. The apparatus ofFIG. 11 further comprises a beam expander 73, a lens system indicated by77, a projection lens indicated by 75 and a light integrator pipeindicated by 76. There is further provided a flat panel display 45. Thebeam expander converts the laser output beam indicated by 1701 into theexpanded beam indicated by 1702. The beam emerging from the ESBG isindicated by 1703 c. The ESBG despeckler device, which is not shown indetail, may be based on any of the ESBG despeckler devices discussedabove. The ESBG despeckler device may be an array of selectivelycontrollable cells as discussed above. In certain cases the ESBGdespeckler device may comprise a single cell. The ESBG despeckler devicemay comprise a stack of similarly configured ESBG arrays or singlecells. The ESBG despeckler device may include ESBG arrays designed tooperate on red, green or blue light. The ESBG despeckler device maycomprise arrays disposed adjacent to each other in a plane. In FIG. 11the beam 1703 c corresponds to a portion the light emitted from a singlecell of the ESBG despeckler device. The lens system 77 transforms thebeam 1703 c into the beam 1703 d forming an illumination patch at theaperture of the light integrator pipe. The integrator pipe emits light1703 e towards the flat panel display. It should be noted that furtherlens elements may be inserted at any point in the optical trainillustrated in FIG. 11 for the purpose of beam illumination profileshaping and aberration correction. The projection lens collects theimage light indicated by 1704 from the flat panel display and focuseslight indicated by 1705 to form an image at the screen 5. The lasersource 1 comprises at least a single laser emitter die. Typically thelaser source comprises separate red, green and blue die. Alternatively,each of the red, green and blue lights may be provided by arrays of die.The invention is not restricted to any particular laser sourceconfiguration. The ESBG drive electronics are not illustrated. The flatpanel display may be an LCD or any other type of device commonly used invideo projection. The apparatus may further comprise relay optics, beamfolding mirrors, light integrators, filters, prisms, polarizers andother optical elements commonly used in displays

In the embodiments of the invention discussed above the ESBG despecklerdevice is located in the illumination path leading up to the flat paneldisplay. In alternative embodiments of the invention the ESBG despecklerdevice may be located in the optical train after the flat panel display.Another laser display according to the principles of the invention isshown in the side elevation view of FIG. 12. The laser display comprisesa laser source 1 and an Electrically Switchable Bragg Grating (ESBG)device 2, which is disposed along the laser beam path. The apparatus ofFIG. 12 further comprises a beam expander 73 a lens system indicated by78 and a projection lens indicated by 75. There is further provided aflat panel display 45. The beam expander converts the laser output beamindicated by 1701 into the expanded beam indicated by 1702. The ESBGdespeckler device, which is not shown in detail, may be based on any ofthe ESBG despeckler devices discussed above. The ESBG despeckler devicemay be an array of selectively controllable cells as discussed above. Incertain cases the ESBG despeckler device may comprise a single cell. TheESBG despeckler device may comprise a stack of similarly configured ESBGarrays or single cells. The ESBG despeckler device may include ESBGarrays designed to operate on red, green or blue light. The ESBGdespeckler device may comprise arrays disposed adjacent to each other ina plane. In FIG. 10 the beam 1703 a corresponds to the light emittedfrom a single cell of the ESBG despeckler device. The lens 78essentially functions as a Fourier transform lens directing lightindicated by 1707 towards a Fourier plane indicated by 1709. The ESBGdespeckler device is disposed in close proximity to the Fourier plane.Desirably, the aperture of ESBG coincides with the illumination patchformed at the Fourier plane by the lens 78. The basic principles ofFourier optics are discussed in a book entitled “Introduction to FourierOptics” by Joseph Goodman published by McGraw-Hill (2^(nd) EditionJanuary 1996). The projection lens collects the image light indicated by1708 from the flat panel display and focuses light indicated by 1705 toform an image at the screen 5. The laser source 1 comprises at least asingle laser emitter die. Typically the laser source comprises separatered, green and blue die. Alternatively, each of the red, green and bluelights may be provided by arrays of die. The invention is not restrictedto any particular laser source configuration. The ESBG drive electronicsare not illustrated. The flat panel display may be an LCD or any othertype of device commonly used in video projection. The apparatus mayfurther comprise relay optics, beam folding mirrors, light integrators,filters, prisms, polarizers and other optical elements commonly used indisplays.

In one embodiment of the invention in which the ESBG despeckler deviceis located after the flat panel display the ESBG despeckler device formspart of the projection lens. Such an embodiment is illustrated in theschematic side elevation view of FIG. 13. In FIG. 13 the projection lensis represent by the elements 75 a,75 b. The ESBG despeckler device isdisposed between lens elements 75 a,75 b. Desirably the ESBG despecklerdevice is position adjacent to the aperture stop. Light 1703 from theflat panel display 45 forms a beam 1704 inside the lens. The lightindicated by 1705 emerging from the projection lens forms an image on aprojection screen. Certain types of projection lenses are designed withexternal aperture stops located in front of the lens that is between thelens and the display panel. In such lens configurations the ESBGdespeckler device would likewise be disposed in front of the lens nearthe external aperture stop. It will be appreciated that the lens shownin FIG. 13 is greatly simplified for the purposes of explaining theinvention. In general the projection lens will be a complexmulti-element system.

FIGS. 14-17 shows schematic front elevation views of differentconfigurations of the ESBG despeckler device in which the ESBGs areconfigured as arrays of selectively controllable ESBG pixels.

In the embodiment of FIG. 14 the ESBG comprises an array containing ESBGpixels such as the one indicated by 24. The pixels may be configured asvariable subwavelength gratings, diffusers or axicons.

In the embodiment shown in FIG. 15 the ESBG despeckler device comprisesa stack of three layers indicated by the symbols R, G, B where thelayers have substantially the same specifications but are designed tooperate on red, green and blue light respectively. The arrays indicatedby R, G, B contains ESBG pixels such as the ones indicated by 25 a,25b,25 c respectively. The pixels may be configured as variablesubwavelength gratings, diffusers or axicons.

In the embodiment of FIG. 16 the ESBG despeckler device comprises astack of three layers indicated by the symbols X, Y, Z containing ESBGpixels such as 26 a,26 b,26 c respectively. The layer indicated by thesymbol X comprises variable diffusers. The layer indicated by the symbolY comprises variable subwavelength gratings. The layer indicated by thesymbol Z comprises variable axicons. It will be clear that manyvariations of the embodiment of FIG. 16 are possible using differentcombinations of ESBG types as well as ESBGs configured for specificwavelengths. The number of layers is only limited by transmission lossesand switching circuitry complexity.

In one embodiment of the invention ESBG arrays may be configuredadjacent to each other as shown in the schematic front elevation view ofFIG. 17. Desirably the ESBG arrays would be mounted on a commonsubstrate. In the embodiments of FIG. 17 separate ESBG arrays areprovided for red green and blue light indicated by the symbols R, G, Brespectively with typical cells in each array being indicated by 27 a,27b,27 c respectively. The embodiments of FIG. 17 will required speciallight guide schemes for delivering light from red green and blue sourcesto the ESBG despeckler device and for combining light into a common beampath after the ESBG despeckler device. It will be clear fromconsideration of FIGS. 15-16 that the principles of stacking ESBG arraysand disposing ESBG arrays on a common substrate as taught above can becombined to provide many different ESBG despeckler deviceconfigurations.

FIG. 18 shows a plan schematic view of one operational embodiment of theinvention for providing colour sequential red green and blue laserillumination. There are provided separated red, green and blue lasermodules. The red module comprises at least one laser source 1R, beamexpansion and collimation lens system represented by 2R, an ESBGdespeckler device further comprising a first ESBG array 3R and a secondESBG array 4R. The lens 2R forms the collimated beam generally indicatedby 1010R. The despeckled beam at the output of the red module isgenerally indicated by 1020R. The green module comprises at least onelaser source 1G, beam expansion and collimation lens system representedby 2G, an ESBG despeckler device further comprising a first ESBG array3G and a second ESBG array 4G. The lens 20 forms the collimated beamgenerally indicated by 1010G. The despeckled beam at the output of theblue module is generally indicated by 1020B. The blue module comprisesat least one laser source 1B, beam expansion and collimation lens systemrepresented by 2B, an ESBG despeckler device further comprising a firstESBG array 3B and a second ESBG array 4B. The lens 2B forms thecollimated beam generally indicated by 1010B. The despeckled beam at theoutput of the blue module is generally indicated by 1020B. A mirror 5Rreflects the red beam along an optical axis to provide a beam 1030R. Agreen reflecting dichroic mirror 5G reflects the green beam along anoptical axis to provide a beam 1030G. A blue reflecting dichroic mirror5B reflects the blue beam along an optical axis to provide a beam 1030G.A lens system generally indicated by 6 directs the beams1030R,1030G,1030B towards a display panel 7. A projection lens 8projects an image of the display panel onto a screen, which is notshown.

In one embodiment of the invention based on the embodiment illustratedin FIG. 18 the first red, green and blue ESBG arrays may be provided ona first common substrate and the second red, green and blue ESBG arraysmay be provided on a second common substrate.

In one embodiment of the invention based on the embodiment of FIG. 18the ESBG arrays are each configured to operate as variable diffusers asdescribed above. In other embodiments of the invention based on theembodiment of FIG. 18 one of the ESBG arrays may operated as a variablediffuser and the other as a variable sub wavelength grating. In a yetfurther embodiment of the invention based on the embodiment of FIG. 18at least one of the ESBGs may combine the optical functions of avariable diffuser and beam homogenizer.

FIG. 19 shows another operational embodiment of the invention. There isprovided a despeckler comprising a first ESBG array 28 a and a secondESBG array 28 b. There is further provided a Diffractive Optical Element(DOE) 29 a. Said DOE may be a holographic element such as a Bragghologram. Said DOE may be a SBG. The DOE directs off axis incident laserlight 1100A into a direction 1101A normal to the surfaces of the ESBGarrays. The light emerging from the ESBG arrays is emitted in theaverage ray direction 1102A. The direction 1102 a may be substantiallythe same as the ray direction 1101A. Normally, ESBGs require off axisillumination for high diffraction efficiency.

FIG. 20 shows an alternative embodiment of the invention similar to thatof FIG. 19 in which incident light 1103A is substantially normal to thesurfaces of the ESBG arrays. A DOE 29 B is used to deflect the lightaway from the incident light direction in the direction 1104A. The ESBGarrays then deflect light into an average ray direction 1105Asubstantially parallel to the incident light direction 110A.

In one embodiment of the invention based on the embodiment of FIGS.19-20 the ESBG arrays are each configured to operate as variablediffusers as described above. In other embodiments of the inventionbased on the embodiment of FIGS. 19-20 one of the ESBG arrays mayoperated as a variable diffuser and the other as a variable subwavelength grating. In a yet further embodiment of the invention basedon the embodiment of FIGS. 19-20 at least one of the ESBGs may combinethe optical functions of a variable diffuser and beam homogenizer.

Electrode Structure for ESBG Arrays

A method of fabricating an ESBG array for use within the presentinvention will now be discussed. The method is very similar to the onedescribed in a co-pending PCT US2006/043938 filed 13 Nov. 2006, claimingpriority to U.S. provisional patent application 60/789,595 filed on 6Apr. 2006, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENTDISPLAY. Although said PCT is directed at the use of ESBGs astransparent elements for displaying symbolic information, thefabrication methods described therein may be applied directly to thepresent invention.

The process of fabricating an ESBG array according to the basicprinciples of the invention is shown in FIGS. 21 to 24. The first sixsteps are shown in FIGS. 21-22. For the purposes of explaining theinvention an ESBG array comprising a single rectangular shaped ESBG isconsidered. It will be clear from consideration of the drawings that theprocess for fabricated a one or dimensional array will require identicalsteps.

Step 1 is illustrated by the plan view of FIG. 21A and the sideelevation view of FIG. 22A. In Step 1 a substrate coated on one sidewith an anti reflection coating 102 and coated on the opposing side witha layer of Indium Tin Oxide (ITO) 103 is provided. The element shown inFIG. 21A and FIG. 22A is referred to as the electrode plate. Only theITO coated surface is shown in FIG. 21A. The antireflection coating maynot be required in certain embodiments of the invention.

Step 2 is illustrated by the plan view of FIG. 21B and the sideelevation view of FIG. 22B. In Step 2 portions of the ITO on theelectrode plate are removed to provide a patterned ITO region generallyindicated by 130 and comprising the ESBG array pixel pad 131, anelectrical connection path 132 and a power supply connector pad 133. Atthis stage in the process the alignment markers 111,112 may be depositedonto the substrate.

Step 3 is illustrated by the plan view of FIG. 21C and the sideelevation view of FIG. 22C. In Step 3 a layer of UV absorbing dielectricmaterial 104 is deposited over the electrode layer 130.

Step 4 is illustrated by the plan view of FIG. 21D and the sideelevation view of FIG. 22D. In Step 4 a portion of said UV absorbingdielectric material overlaying ESBG array pixel pad 131 is removed. FIG.21E shows a plan view of the superimposed dielectric layer and ITOlayer.

At Step 5, which is not illustrated, a second substrate again coated onone side with an anti reflection coating and coated on the opposing sidewith a layer of ITO is provided. The antireflection coating may not berequired in certain embodiments of the invention.

Step 6 is illustrated by the plan view of FIG. 21F and the sideelevation view of FIG. 22E. In Step 6 the ITO layer of said secondsubstrate is etched to provide the electrode structure general indicatedby 170 comprising a central portion 171 substantially identical to andspatially corresponding with the ESBG array pixel pad 131, thebackground area 172 and the perimeter regions 173 a, 173 b from whichITO material has been removed. The width of the perimeter regions 173 a,173 b are required to be large enough to avoid the risk of shortcircuits occurring. Desirably the width of the perimeter regions shouldbe less than 50 microns.

In a further step, Step 7, which is not illustrated, the two substratesprocessed according to the above steps are combined to form a cell withthe electrode coated surfaces of the two substrates aligned in opposingdirections and having a small separation.

In a further step, Step 8, which is not illustrated, the cell is filledwith a PDLC mixture.

In the final step, Step 9, of the fabrication process the HPDLC regioncorresponding to the ESBG array pixel pad is recorded.

A schematic side view of an assembled ESBG array cell according to thebasic principles of the invention is shown in FIG. 23. Again, only onepixel pad of the array is shown for simplicity. The ESBG array comprisesa first transparent substrate 101, an antireflection coating 102, afirst ITO layer 130 covering a portion of the surface of the substrate,a UV absorbing dielectric layer 140 covering a portion of the ITO and ofthe substrate, a PDLC layer 110, a second substrate 105 having onesurface coated with an ITO pattern indicated by 170 and the opposingface coated with an anti reflection coating 106. The antireflectioncoatings 102,106 may not be required in certain embodiments of theinvention.

FIG. 24 shows the HPDLC recording process used. The cell is illuminatedfrom one side by a pair of intersecting beams generally indicated by1000 from a UV laser. The incidence angles of the beams will be precisebeam angle requirements of the illuminator device. The intersectinglaser beams interfere only in the region of PDLC under the aperturesetched out of the dielectric layer. As described earlier theinterference causes a grating 300 comprising alternatingLC-rich/polymer-depleted and LC-depleted/polymer-rich regions to beformed. At the same time, the PDLC material is UV cured by illuminatingthe cell from the opposite side using incoherent UV light generallyindicated by 2000. The incoherent UV light gives rise to the PDLC region300. The PDLC is characterized by large LC droplets having randomorientations. However, the HPDLC grating is characterized by tinydroplets having a preferred alignment. The relative intensities of theUV laser and the incoherent UV source are balanced to optimize theswitching characteristics of the PDLC and HPDLC regions. When anelectric field source is coupled across the ITO electrodes 130 and 170the grating remains active when no field is applied but is deactivatedwhen a field is applied.

For the purposes of explaining the invention the thicknesses of thecoatings in FIGS. 21-24 have been greatly exaggerated. The details ofthe wiring around the pads and the means of connecting the pad to thepower supply have not been shown in FIGS. 21-24. Although FIGS. 21-24show only one ESBG pixel pad, the process steps may be applied to anarray of ESBG pixels arrayed on large area substrates, such ascommercially available seven inch substrates. Although the ESBG pixelpad shown in FIGS. 21-24 is of rectangular shape, the process maygenerally be applied to ESBGs of any required shape and size.

A method of fabricating a ESBG array in accordance with the inventionwill now be described with reference to FIG. 25.

At step 500, a substrate to which a transparent electrode layer has beenapplied is provided.

At step 501, portions of said transparent electrode layer are removed toprovide a patterned electrode layer including at least one ESBG pixelpad.

At step 502,a layer of UV absorbing dielectric material is depositedover said patterned electrode layer.

At step 503, the portion of said UV absorbing dielectric materialoverlapping said ESBG pixel pad is removed.

At step 504,a second substrate to which a transparent electrode layerhas been applied is provided.

At step 505, the transparent electrode layer of said second substratelayer is etched to provide a patterned electrode layer including anelectrode element substantially identical to and spatially correspondingwith said ESBG pixel pad.

At step 506, the substrates are combined to form a cell with the coatedsurfaces of the two electrode coated surfaces aligned in opposingdirections and having a small separation.

At step 507, said cell is filled with a PDLC mixture.

At step 508, the cell face formed by the first substrate is illuminatedby crossed UV laser beams, and simultaneously illuminating the cell faceformed by the second substrate by an incoherent UV source.

In production, the masks will need to be mirror imaged and coloredappropriately for the particular process and photo-resist used. The toplevel ITO mask would typically include a set of alignment features suchas the ones shown in FIG. 24 to facilitate the assembly of the ESBGarray. Further alignment features may be incorporated if required by theprocess.

The ITO layer typically has a coating resistance of typically 300-500Ohm/sq. A typically example of an ITO film used by the inventors is theN00X0325 film manufactured by Applied Films Corporation (Colorado).Typically, the ITO film has a thickness of 100 Angstrom. Typically, theITO film is applied to 0.7 mm thickness 1737F glass. The ITO layer 170should have the same properties as the ITO of Level 1.

The dielectric layer 140 in FIG. 23 should have a thickness sufficientto withstand a peak voltage of 100V between the ITO layers. Desirably,the dielectric should be free from pinholes. The transmission of thedielectric layer at a wavelength of 365 nm and incidence angle in therange 30 to 60 degrees should, ideally, be less than 0.1%. However, inmany applications transmissions of up to 5% may be acceptable.

Typically the layer-to-layer registration should be ±25 micron (±0.001inch).

A first benefit of the process discussed above is that it eliminates theneed for a focused mask in the exposure set-up. In mask-based exposureprocesses the grating area would need to be slightly larger than theactual ESBG pixel in order to improve background clarity. The use of anetched UV absorbing dielectric layer as disclosed in the presentapplication allows more readily achievable production tolerances,simplifying mass production and lowering cost. A second benefit of thedisclosed fabrication process is that it provides an extremely clearbackground, which is highly desirable in illumination applications. Animportant feature of an ESBG array fabricated using the above process isthat the HPDLC is localized to the ESBG array pixels. The ESBG arrayfabrication method described above results in a more efficient and costeffective mass production process.

The present invention does not assume any particular process forfabricating ESBG despeckler devices. The fabrication steps may becarried out used standard etching and masking processes. The number ofsteps may be further increased depending on the requirements of thefabrication plant used. For example, further steps may be required forsurface preparation, cleaning, monitoring, mask alignment and otherprocess operations that are well known to those skilled in the art butwhich do not form part of the present invention

Although ESBG electrode patterning methods for use with the presentinvention have been discussed in relation to uniformly patternedtwo-dimensional arrays such as the array illustrated in FIG. 25 where anESBG array 171 comprises square ESBG pixels such as 172 it will be clearthat the invention may be applied using electrodes patterned in morecomplex geometries. For example, FIG. 26 shows a plan view of anelectrode structure 173 that provides a non-uniform ESBG array patternfor use in a dual electrode structure such as the one illustrated inFIG. 23. The electrode elements such as 174 have very fine gaps toeliminate super grating effects. Typically, the gaps are approximatelyfive microns. From consideration of FIG. 23 it will be appreciated thatthe electrode elements are energized from underneath by electricalconnections to second layer electrode drive tracks. The first and secondelectrodes sandwich ESBG layers of shape defined by the electrodeelement shapes. Alignment features such as the one indicated by 175 maybe provided.

In one embodiment of the invention that uses an electrode structure ofthe type shown in FIG. 26 the ESBG regions sandwiched between theirregular electrode elements may encode clusters of point sources foruse in angular diversity despeckling.

A Preferred Angular Diversity Despeckler Embodiment

The preferred embodiments of the invention will now be discussed withreference to the drawings in FIGS. 28-41. The ESBG elements are designedto function as diffusers providing speckle reduction according to theprinciple of angular diversity.

One particular embodiment of the invention that uses angular diversityspeckle reduction is illustrated in the schematic side elevation view ofFIG. 28. The apparatus comprises a laser source 1, a beam expandercomprising the lens elements 91,92 a despeckler further comprising theESBG elements 93,94, a Diffractive Optical Element (DOE) 95, a lens 96 aflat panel display 97 and a projection lens 98.

The first ESBG element 93 is a plane Bragg grating in other words agrating in which the Bragg surface vectors are aligned in a commondirection such that a collimated input beam in a first direction isdeflected into a collimated beam in a second direction. The second ESBGelement 94 comprises an array of ESBG elements. Advantageously, the ESBGelements and the DOE which are shown as separated in FIG. 28 form asingle laminated element. As will be explained below, the second ESBGelement provides a multiplicity of narrow beams, referred to asbeamlets, separated by small angles where each beamlet is associatedwith a unique ESBG array pixel.

Each ESBG array pixel may be understood to be a plane gratingcharacterized by a unique grating vector or a grating vector selectedfrom a set of predetermined grating vectors. The angles of separation ofthe beamlets are referred to as Inter Beamlet Angle (IBA). In certainembodiments of the invention said ESBG elements may incorporate opticalpower to control the IBA and individual beamlet divergence angles. Theeffect of incorporating optical power into the ESBG array pixels isequivalent to disposing a microlens array in series with the ESBG 94.The ESBG arrays may encode further optical properties for optimizing theoptical characteristics of the beamlets. For example, in furtherembodiments of the invention the ESBG arrays may encode diffusingcharacteristics. In yet further embodiments of the invention the ESBGarrays may encode keystone correction.

FIG. 29 illustrates the operation of the ESBG element 93, 94 in moredetail. For convenience the ESBG elements 93,94 are illustrated asseparated single pixel elements. Normally, ESBGs require off axisillumination for high diffraction efficiency. The incident light 1102 issubstantially normal to the surfaces of the ESBG elements. The ESBGelement 93 deflects the light away from the incident light direction inthe direction 1102B. The ESBG element 94 then deflects light 1102B intoan average ray direction 1103 substantially parallel to the incidentlight direction 1102

The purpose of the DOE element 95 is to modify the intensity profile ofthe illumination light to generate a flat average intensity profile atthe flat panel display. Typically, the output light from the laser willexhibit a Gaussian intensity profile. A further function of the DOEelement 95 may be to apply a predetermined amount of diffusion to theillumination light. However, as indicated above the diffusion mayinstead be provided by one of the ESBG elements.

As illustrated in FIG. 28 the laser provides a collimated output beam1100 which is expanded into a diverging beam 1101 by the lens 91 andthen re-collimated into the beam 1102 by the lens 92. The beam 1102 isdiffracted by the ESBG elements in turn providing a collimated beam1102. The beam 1102 passes through the DOE 95, which changes the spatialintensity profile of the beam providing an output beam 1104. The lens 96focuses the beam 1104 into the converging beam 1105, which forms anillumination patch at the surface of the flat panel display. Theprojection lens 98 then projects an image of the microdisplay onto aremote screen. The invention is not restricted to any type of projectionlens. The ray paths after the flat panel display are not illustratedsince the invention is not restricted to any particular method ofdisplaying an image. In certain applications of the invention the imagedisplayed on the flat panel display may be viewed by means of aneyepiece as used in, for example, a wearable display.

A method of recording the ESBG array is illustrated in the schematicside elevation view of FIG. 30. The recording apparatus comprise a lasersource 201 a beam expander lens system comprising the lenses 251,252 abeam splitter 253, a mirror 254, a second lens 255, a computer generatedhologram (CGH) 256 and a cell 257 containing the HPDLC mixture intowhich the SBG is recorded. The mirror 254 is typically a planar element.In certain embodiments of the invention it may be advantageous to use acurved mirror or a diffractive mirror encoding optical power in order tocontrol the geometrical characteristics of the light reflected from themirror. The beam splitter may be a beam splitter cube or a flat plate towhich a beams splitter coating has been applied. The CGH 256 is designedto generate a set of beamlets of the type described above from a singleinput beam. As illustrated in FIG. 28 the laser provides a collimatedoutput beam 1204 which is expanded into a diverging beam 1201 by thelens element 51 and then re-collimated into the beam 1202 by the lens252. The invention does not rely on any particular method of expandingand collimating the light from the laser. The arrangement shown in FIG.30 is suitable for lasers providing collimated output. Solutions forcollimating and expanding the laser beam where the laser provides adivergent beam output are well know to those skilled in the art of laseroptics. The beam splitter 253 divides the beam 1202 into the reflectedbeam 1203 and a transmitted beam 1205. The reflected beam 1203 isreflected by the mirror 254 to provide the beam 1024 incident on theESBG cell 257. The lens 255 converts the transmitted beam 1205 into thebeam 1206 incident on the CGH. The CGH is designed to convert theincident beam 1206 into the fanned out beamlets generally indicated by1207. A ESBG is recorded by exposing a HPDLC mixture contained in thecell to the simultaneously applied beams 1204 and 1207 and patterningthe cell with transparent array of electrodes according to theprinciples discussed above. In holographic recording terms beam 1204provides the reference beam and beam 1207 provides the object beam. Fromconsideration of FIG. 30 it will be apparent that after reconstructionusing the reference beam the resulting ESBG element is equivalent to aN×N array of sub elements or pixels where each pixel encodes onebeamlet. Typically the array would have dimension N=20.

Each beamlet contributes a separate speckle pattern. The cumulativeeffect of combining the full set of speckle patterns provided by thebeamlets is to reduce speckle contrast and hence reduce the magnitude ofthe speckle perceived by a viewer. The physical mechanism by which thespeckle is reduced relies on the angular diversity resulting fromcombining many beamlets characterized by small IBAs. The ESBG arraypixels are modulated by selectively applying a voltage waveform to eachESBG pixel. By providing a sufficiently large dimension N and modulatingthe ESBG pixels using a suitable waveform a large number of specklepatterns may be averaged within the eye integration time. The appliedwaveform at any given ESBG pixel may have a range of possiblecharacteristics such as rectangular or triangular and may be regular orrandom. The inventors have found that approximately 27 speckle patternsmust be integrated to reduce the intensity variation along line in thespeckle pattern to 1%. Assuming a 1/60 second eye integration time thisgives 27×60=1620 speckle samples per second.

An exemplary monochromatic despeckler embodiment would have thefollowing specifications: operating wavelength: 550 nm; an inter beamangle of 0.2 degree giving a total angle for a 40×40 width of 39*0.2=7.8degrees. The size of the ESBG elements is typically 50×50 mm. The DOEelement 95 would typically provide isotropic diffusion over a 2.5-degreecone.

FIGS. 31-32 illustrate an alternative method of recording the ESBG arrayto the one illustrated in the schematic side elevation view of FIG. 30.FIG. 31 is identical to FIG. 30 and is shown again for comparisonpurposes. FIG. 32A is a schematic side elevation view of an alternativeembodiment in which the recording apparatus again comprises a lasersource 201 a beam expander lens system comprising the lenses 251,252 abeam splitter 253, a mirror 254, a second lens 255, a computer generatedhologram (CGH) 256 and a cell 257 containing the BPDLC mixture intowhich the SBG is recorded. However in the case of FIG. 32A the CGH 56generates four fan out beams instead of a single beam as used in FIG.31. FIG. 32B is a front elevation view indicated the effective originsof the beams near to the output surface of the CGH 258 with the originof one beam being indicated by the symbol 259.

A further embodiment of the invention illustrated in the schematic sideelevation view of FIG. 33A is similar to the embodiment of FIG. 32.Again the first ESBG 93 is a plane Bragg grating in other words agrating in which the Bragg surface vectors are aligned in a commondirection such that a collimated input beam in a first direction isdeflected into a collimated beam in a second direction. The second ESBG94 comprises an array of ESBG elements. However, in the case of FIG. 33Athe DOE diffuser 93 is disposed in front of the ESBG elements.Advantageously, the ESBGs and the DOE form a single laminated element.As indicated in FIG. 33B the ESBG elements are again configured tooperate in the fashion illustrated in FIG. 29

In an alternative embodiment of the invention illustrated in theschematic side elevation view of FIG. 34 the ESBG despeckler device isalso based on two ESBG elements. Again the first ESBG element 93 is aplane Bragg grating in other words a grating in which the Bragg surfacevectors are aligned in a common direction such that a collimated inputbeam in a first direction is deflected into a collimated beam in asecond direction. The second ESBG element 96 comprises an array of ESBGpixels, which operate according to the principles of the ESBG array 94discussed above. However, the ESBG element 96 now encodes diffusioncharacteristics eliminating the need for the DOE diffuser 95.Advantageously, the ESBG elements form a single laminated element. Asindicated in FIG. 34B the ESBG elements 93,96 are configured tooperation in the same fashion as the ESBG elements 93,94 illustrated inFIG. 29 and are referenced using the same symbols.

Advantageously, in the embodiments of FIGS. 28-34 the ESBG array isfabricated by first designing and fabricating a CGH with the requiredoptical properties and then recording said CGH into the ESBG element.Recording the CGH into the ESBG element essentially means forming ahologram of the CGH using conventional holographic recording techniqueswell known to those skilled in the art of holography. It should be notedthat the resulting ESBG element is not identical in every respect to theCGH since properties of a CGH rely on its surface phase relief featureswhile the optical characteristics of a Bragg grating such as an ESBGrely on a complex three dimensional fringe distribution

A volume hologram as an ESBG has much a much higher SBWP (Space BandWidth Product) than a surface relief CGH since any point in the hologramcan take a specific phase value. In the case of a CGH the entire opticalfunctionality burden must be carried by just two phase levels. The CGHis calculated or iteratively optimized via a direct or iterative FastFourier Transform (FFT) algorithm where the reconstruction occurs in thefar field. Advantageously, there are very many different solutions to agiven intensity diffraction pattern problem. Hence the speckle grains inthe reconstructed image can be changed, without changing the overallintensity image. There are many ways to implement such a phase change.For example: a) by recalculating the CGH with a different phase on theobject; b) by recalculating the CGH with a different algorithm, ordifferently tuned algorithm; c) by adding a constant or slightlyrandomized phase carrier on the CGH; or d) by reverting the phase pixelswhere the CGH operates according to a binary or Babinet principle.

Embodiments Using ESBG Arrays Operating in Anti Phase

In an one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 35A there is provided a display device comprisinga laser source 1, a beam expander comprising the lens elements 91,92 adespeckler further comprising the HOE 93, the ESBG arrays 99A,99B, alens 96 a flat panel display 97 and a projection lens 98. The ESBGelements 99A, 99B are each arrays of selectively switchable ESBGelements. Each ESBG array operates according to the principles of theESBG arrays of FIGS. 28-34. Each ESBG array essentially provides amultiplicity of beamlets separated by small angles. Each ESBG array alsoprovides a predetermined amount of diffusion to each said beamlet.

The HOE 97 is typically recorded in a photopolymer of the typemanufacture by DuPont. Desirably, the HOE has a diffraction efficiencyof at least 99%. The HOE typically diffracts incident collimated lightat normal incident into a direction at 30 degrees to said normalincidence direction. The output angle of the HOE provides the off axislaunch angle for the ESBGs. The invention does not rely on anyparticular value of the launch angle. However the inventors have foundthat typical launch angles are in the range 30-50 degrees. The apparatusof FIG. 35A may further comprise a DOE for providing illuminationcontrol functions of the type discussed above. As indicated above theneed for the DOE may be eliminated by providing suitable diffusion andother illumination control optical functions within the ESBGs.

Advantageously the ESBG arrays are offset by a fraction of the ESBGelement width in at least one of the vertical or horizontal array axes.In some cases the ESBGs may be offset by an ESBG element width in atleast one of the vertical or horizontal axes.

The configuration of the HOE 93 and the ESBG arrays 99A,99B isillustrated in FIG. 35B. In other embodiments of the invention the HOEmay be replaced by another diffractive device suitable for performingthe required beam steering such that each ESBG elements diffracts lightincident at some specified launch angle into a direction normal to thesurfaces of the ESBG elements. It will be clear from consideration ofFIGS. 35A-35B that and equivalent arrangement of the HOE and the ESBGarrays is provided by disposing the HOE between the first and secondESBG arrays. In other embodiments of the invention the ESBG elements maybe tilted at a suitable angle with respect to the illuminationeliminating the need for the HOE 97.

In the embodiment of FIG. 35A the ESBG arrays 99A,99B are driven in arandom anti-phase fashion by means of an ESBG controller which is notillustrated. To put it another way the ESBGs 99A,99B are operated intandem with alternating voltages applied across the ESBG layers. Theoptical effect of each ESBG despeckler device is varied from zero tomaximum value at a high frequency by applying an electric field thatvaries in a corresponding varying fashion. Each incremental change inthe applied voltage results in a unique speckle phase cell.

It should be noted that since the ESBG arrays are driven in anti-phaseonly one ESBG element is active at any time along a give ray paththrough the ESBG arrays.

Referring to FIG. 35C which is a chart showing voltage versus timeapplied to the ESBG arrays 99A and 99B it will be seen that there is aphase lag between the voltages 1001,1002 applied across the ESBG arrays.The effect of applying such waveforms is that the average intensity 1003of the speckle phase cells remains substantially constant, therebysatisfying the statistical requirements for speckle reduction. Othertypes of waveforms may be applied, for example sinusoidal, triangular,rectangular or other types of regular waveforms. Alternatively, it maybe advantageous in statistical terms to use waveforms based on a randomstochastic process such as the waveforms 2001,2002 illustrated in thechart of FIG. 35D. Again the effect of applying such waveforms is thatthe average intensity 2003 of the speckle phase cells remainssubstantially constant.

FIG. 36 is a schematic side elevation view of a further embodiment ofthe invention similar to the one shown in FIG. 35. The apparatus of FIG.36 further comprises an electrically controllable phase modulator cellindicated by 46. The phase modulator is any optical device that canprovide a phase retardation in the range from 0 two pi radians. Theinvention is not limited to any particular phase modulator. Desirably,the phase modulator may be based on a ESBG despeckler devices whichencodes a sub wavelength grating. By providing phase diversity andangular diversity the apparatus of FIG. 36 offers an effective solutionfor reducing both near and far field speckle.

Embodiment Using Combined Phase Diversity and Angular Diversity

In a further embodiment of the invention illustrated in the schematicside elevation view of FIG. 38 there is provided a despeckler apparatusbased on the principle of angular and phase diversity. The apparatuscomprises a plane ESBG indicated by S1 for deflecting normally incidentcollimated light 3201 through a specified angle to provide a beam 3204and a second plane ESBG S2 design to diffract incident light 3204 atsaid angle into direction 3205 normal to S2 When the ESBGs S1, S2 arenot in their diffracting states incident light 3201 is transmittedwithout substantial deviation through S1 as the light 3202 and the light3202 is in turn transmitted through S2 without substantial deviationemerging as light 3203. When the ESBG S2 is not in a diffracting statethe incident light 3204 is transmitted without deviation in thedirection indicated by 3206.

The lateral displacement of the beam when the ESBGs are in a diffractingstate results in an optical path difference given by the product of theseparation of gratings S1,S2, the average refractive index of theoptical path between gratings S1,S2 and the tangent of the diffractionangle. In effect the apparatus of FIG. 38 provides angular diversity andphase diversity simultaneously. ESBG elements illustrated in FIG. 38 mayform pixels of two-dimensional ESBG arrays.

FIG. 39 is a schematic side elevation view of a further embodiment ofthe invention related to the embodiment illustrated in FIG. 37A. Theapparatus comprises a plane ESBG indicated by S1 for deflecting normallyincident collimated light 3210 and 3220 through a specified angle toprovide beams 3211,3221 respectively and a second plane ESBG S2 designto diffract incident light such as 3221 at said angle into direction3222 normal to S2. A third plane grating ESBG S3 diffracts lightincident at said diffraction angle such as 3211 into a direction 3212normal to S2. When the ESBGs S1, S2, S3 are not I their diffractingstates incident light 3210, 3220 is transmitted without substantialdeviation through S1,S2,S3 in turn emerging as light 3213,3223respectively. When the ESBG S2,S3 are not in a diffracting state thediffracted light 3211,3221 is transmitted without deviation throughS2,S3 emerging as the light 3214,3224 respectively. It will be clearfrom consideration of FIG. 39 that the lateral displacement of theincident light when the ESBGs are in a diffracting state results in anoptical path difference given by the product of the separation ofgratings S1,S2 or S2,S3, the average refractive index of the opticalpath between gratings S1,S2 or S2,S3 and the tangent of the diffractionangle. In effect the apparatus of FIG. 38 provides angular diversity andphase diversity simultaneously. The ESBG elements illustrated in FIG. 38may form pixels of two-dimensional ESBG arrays. It will be clear fromconsideration of FIG. 39 that a range of switching schemes may beapplied to the ESBG layers to provided combined phase and angulardiversity speckle reduction.

Recording of Anti-Phase ESBG Arrays

In preferred embodiments of the invention a multiplicity of differentdiffuser pattern are recorded in a master diffractive element such as aCGH. Said multiplicity of different diffuser patterns are then recordedinto each of two ESBG arrays. Desirably, the ESBGs are operatedaccording in random anti phase according to the principles discussedabove.

The recording principles for ESBG arrays designed to operate in randomanti phase are illustrated schematically in FIGS. 40-41. In a first stepshown in FIG. 40 a quartz binary CGH diffuser indicated by 256 recordedusing an optical arrangement similar to the one illustrated in FIG. 32is provided. In the next step also illustrated in FIG. 40 a CGH diffuseris used in a holographic recording process to form two ESBG cellsindicated by 320,330 containing ESBG elements such as the ones indicatedby 321,331. A 10×10 pixilated electrode pattern is then deposited oneach cell so that 100 individual sub-cells are provided. The second ESBGarray is identical to the first ESBG array but is rotated through 180degrees such that the ESBG elements indicated by 322,323 in the firstarray become the elements indicated by 332,333 respectively in thesecond array. The first and second ESBG arrays operate in random antiphase as described earlier. At any time all the cells are diffusing.However, the electrode activation pattern is randomly generated;ensuring a different phase and therefore a different speckle pattern isconstantly generated. The electrode pattern would typically be updatedat a frequency of around 2.5 kHz.

In FIG. 40 the diffuser 310 is an ESBG characterized by a uniformdiffusion prescription modulated using a pixilated structure. In otherwords each pixel is characterized by the same diffusion characteristics.The number of possible speckle patterns can be greatly increased byrecording a master array of CGH elements with pre-computed diffuserprescriptions mapped to the individual pixels in the ESBG arrays. TheCGH elements may provide angular and phase diversity. FIG. 41 is a frontelevation view of such a CGH array indicated by 340 and comprising CGHelements such 341.

Although a regular 10×10 ESBG is illustrated in FIGS. 40-41 it will beclear that arrays of much higher resolutions may be fabricated based onthe principles discussed above. However, small size, cost and complexityrequirements in certain despeckler applications may limit the number ofelements in the array. It will also be clear that irregular electrodepatterning such as that shown in FIG. 27 may be used.

The master array may comprise a wide range of different diffusers whoseprescriptions may be designed to provide diffusion patternscharacterised by scattering angles, scattering pattern asymmetries,structure diffusion patterns and many others. The invention is notrestricted to any particular type of diffusion pattern. Typically, theinventors have found that the far field diffusion patterns required inpractical despecklers require a total diffusion angle in the region of0.5 degrees. The diversity of the available diffusion patterns which mayprovide angular phase and polarization diversity results in a very largenumber of speckle samples for integration

The despeckler relies on combining the effects of many different typesof diffuser patterns encoded within a diffractive element. The diffuserpatterns may rely on angular diffusion patterns for providing angulardiversity with an effect similar to that of a rotating ground glassdiffuser.

Embodiments Based on Hadamard Diffusers

Approaches to speckle reduction based on diffusers suffer from theproblem that assigning random phases to each speckle cell will require alarge number of phase patterns to achieve the maximum theoreticalspeckle reduction. To overcome this problem in one embodiment of theinvention the ESBG mays may be configured to provide Hadamard diffusers.The principles of Hadamard phase plates are well known in the opticaldata processing literature. The theory of Hadamard diffusers in relationto speckle reduction is discussed in some detail in a paper by J. I.Trisnadi entitled “Hadamard speckle contrast reduction,” (Optics Letters29, 11-13 (2004)). Hadamard diffusers offer the advantage of a shortphase correlation length allowing the target speckle diversity to beachieved more easily. By providing the permutations of rows and columnsaccording to Hadamard theory a set of N² Hadamard phase patterns isgenerated providing considerable economy in terms of the number of phasepatterns with a prescribed combination of pi and 0 radian phase shifts.When these phase patterns are presented within the eye integration timewith equal weight N² independent speckles are produced resulting inspeckle contrast reduction by a factor of N. The corresponding classicalN×N diffuser using random phase would in theory require an infinitenumber of phase patterns to achieve the same speckle contrast.

Passive Matrix Addressing Schemes for Embodiments Using ESBG Arrays

Several of the embodiments disclosed in the present application requirean active matrix switching scheme. The passive matrix addressing schemesto be used in the present invention differs from the ones traditionallyused in display panels such as LCD panels. In the latter case all thecomplexity and requirement for well-defined and steep transitionresponses is dictated by the way in which the matrix addressing processmust be implemented. In normal passive matrix addressing a voltage isdefined for all pixels along a first line and a line scan pulse isasserted. When the first line has been scanned the same procedure isapplied for the next line and so on. The RMS voltage applied to eachpixel is essentially overdriving when scanning a given line, but only bya sufficient margin to ensure that adjacent pixels are not activated,and relying on the slower decay to ‘hold’ state until the next scancomes round. Such a procedure is required in any imaging display whereit is necessary to address any specific random pixel in the arraywithout affecting all other pixels.

In the case of the ESBG arrays used in the present invention the row andthe columns are driven with arbitrary, random bit patterns as will beexplained in the following paragraphs. In other words there is norequirement for a scan drive as in an active matrix display. This offerscertain advantages, which may be appreciated by considering the simpleexample of a 3×3 array. Only six drivers required for a multiplexeddrive scheme in a 3×3 array. In this case the column drivers couldprogram any one of 2³=8 patterns into the column drive shift register.The row driver could then decide for each row whether to display thatrow as a positive pattern (i.e. drive a zero, so anything set to 1 inthe row is driven) or as a negative pattern (i.e. drive a 1 so anythingset to 0 is driven). There are therefore eight distinct options for therow drive also. This gives a total of 64 patterns. Some examples are:

Column=010 Row=000, in which case a vertical stripe down the centre isdriven;Column=010 Row=111, in which case two outer vertical stripes are driven;Column=010 Row=010, in which case hollow diamond pattern is provided;andColumn=010 Row=101, in which case an X shape pattern is provided; and soon

The invention is certainly not limited to arrays of such low resolution.At the time of filing the present application the inventors believe thatESBG array 240×240 switched pixels or higher resolution of area 20×20 mmare feasible.

In the ESBG array each pixel provides a different hologram prescriptionresulting in a different mix of light for each pixel state and hence adifferent speckle pattern. Both the row and the columns are driven as ifthey were columns, with arbitrary, random bit patterns. In other wordsno scan drive is required as in displays. Therefore there is thesimplification that only one type of driver is required. One usefulfeature in the dual array despeckling scheme is that if just the rowdrive is inverted, typically by sending a single control bit into thedrive chip, then the pattern is inverted. This means that the same datacan be shifted into both ESBG arrays. It would be necessary for onearray to be set to the invert state and the other array to the normalstate.

For example, a 20 bit column driver chip on each sheet of glass candrive a 20×20 array that has 1099511627776 distinct patterns. Even ifthe optical system is symmetric resulting in the need to divide thenumber of patterns by 16 to account for the four axes of symmetry, thenumber of available speckle patterns is still large. Since all thecontrol lines are driven all the time the need for precise voltagecharacteristic control is eliminated. Likewise, the problems ofmaintaining contrast and light efficiency are eased by having the fulldrive voltage applied at all times. Using this method, the rows andcolumns are fully charged or discharged. Hence the problem of crosstalkis avoided. The pattern can be held as long as desired for optimizingthe response time, power dissipation and visual integration of thediffuser descriptions.

FIG. 42 is a schematic three dimensional illustration showing theapparatus for driving an active matrix despeckler array according to theprinciples of the invention. The apparatus comprises a first substrate410 and a second substrate 420, Said first substrate has a first grid ofelectrodes 411 applied to its second surface. Said second substrate hasa second grid of electrodes 421 applied to its first surface whichopposes the second surface of the first substrate. The electrode gridsare oriented in orthogonal directions providing orthogonal rows andcolumns as illustrated in FIG. 42. In contrast to display applicationsin the apparatus of FIG. 43 the rows and columns are driven witharbitrary random bit patterns. There is no scan drive as used in adisplay device. The rows and columns are either full charged ordischarged with the pattern being as long as desired for optimising theresponse time, power dissipation and visual integration of the diffuserdescriptions. There is no significance to the rows and columns andidentical drivers may be used for the two. By this means it is possibleto provide very large numbers of random speckle patterns.

FIG. 43 shows the addressing scheme in more detail within which rowaddressing lines such as X1,X2 and column addressing lines such as Y1,Y2are used to drive pixels such as the one schematically illustrated by430.

The array may use a Chip-On-Glass (COG) mounted device such as the 240channel NT7706 device manufactured by Novatec. Typically a customcontroller, 40 v boost switcher and discrete common drive and softwareincluding the despeckler switching algorithm would also be provided. Thedevice would typically require a 5 volt input. The link to the cellwould be via a standard 10 channel off the shelf flex. Communicationsand power connectors would be mounted COG.

FIGS. 44-45 represent the sequence of 1 or 0 logic states applied torows or columns of a 10×10 pixel array. FIG. 44 represents the sequenceof logic states generally indicated by 441 provided by the row driver.FIG. 45 represents the sequence of logic states generally indicated by442 provided by the column driver. Each driver provides 2¹⁰ randomstates into each of the column and row shift registers. It will be clearfrom consideration of FIG. 41 that the number of distinct patterns thatcan be generated by the above means is given by(2²⁰×2²⁰)/2=1,099,511,627,776.

FIG. 46 is a schematic view of the 3×3 despeckler ESBG array module. Thedevice generally indicated by 460 comprises ESBG elements such as 461, acolumn drive 462 a row driver 463, input data interface indicated by 464and communication link 465. The column driver 463 transfers 2³=8patterns into a column shift register. The row driver 462 transfers 2³=8patterns into a row shift register.

FIG. 47 illustrates the waveform during despeckler ESBG elementoperation. High Voltage (HV) and Ground (GND) potentials are indicatedin each case. The applied voltages are typically between −50 v and −80v. The solid line plot indicated by 201 results from the ESBG elementbeing driven clear, that is the ESBG is in its non-diffracting state.The dashed line plots indicated by 202, 204 result from the ESBG elementremaining in its diffracting state. Note that the polarity of the drivemust be alternated to avoid DC ionization of the HPDLC material.

FIG. 48 illustrates the speckle sample generating process used in FIG.46 in more detail. Column and row data is indicated by the tables470,480 containing logic 1,0 data as indicated by 471,481. The state ofthe ESBG array for the illustrated column and row data is indicated bythe set of array patterns 490 of which the array 491 is one examplecomprising ESBG elements such as 492. For the 3×3 array illustratedthere are 8*8=64 possible patterns. FIG. 49 provides an illustration ofthe complete set of patterns indicated by 50. One pattern in the set isindicated by 451 with one element of the patter 451 being indicated by452. It should be noted that the ESBG element has been shown blackwhenever the row and column voltage are different and white whenever therow and column voltage are the same. It should also be noted that thereare actually only 32 not 64 distinct patterns, since in the case whenthe cell is driven the transition 0-1 is the same as 1-0 and in the casewhen the cell is not driven 11 is the same as 00.

It will be clear that the switching schemes illustrated in FIGS. 42-49may be applied to any of the ESBGs described in the present applicationincluding ESBG arrays operating in random anti phase.

FIG. 50 is a schematic side elevation view of an ESBG despeckler deviceusing ESBG arrays operating in random anti phase, which furthercomprises a polarization switch stage. The apparatus generally indicatedby 90 comprises two ESBG arrays 2A,2B operating in anti phase asdescribed above and a polarisation switch 46. The two diffusing layersoperate in anti-phase triggering an angular/phase diversity due to theanti-phase operation and the 180° recording set-up as described above.The polarization switch provides polarization diversity. Thepolarization switch, which is not pixilated, is recorded as a subwavelength grating. The polarization switch operates as a fastpolarization rotator providing a phase shift for a given input lightpolarization. The polarization switch is randomly switched with respectto the pixelated diffuser. Its phase shift is always set to π, in orderto create the maximum speckle contrast using local destructiveinterference.

Embodiments Providing Edge Illuminated Despecklers

In the embodiments of the invention discussed above the ESBG despecklerdevice has been implemented in an illuminator for use in a conventionalfront or rear projection display. In further embodiments of theinvention the ESBG despeckler devices described above may be configuredwithin edge illuminated illuminators and display. Such embodiments ofthe invention may be used to provide a backlight for illuminating anddespeckling laser illuminated flat panel displays.

In certain cases to be discussed below an edge lit ESBG despecklerdevice may itself provide a complete display device.

In a further embodiment of the invention illustrates in the schematicside elevation view of FIG. 51 there is provided an edge illuminateddespeckler. Referring to FIG. 51 we see that the edge-illuminateddespeckler comprises first and second transparent substrates 81,82 andESBG element 84 sandwiched between the substrates, a transparent region83 adjacent to the ESBG element and a light-coupling element 80. Thelight coupling element may be a diffractive optical element, a prismaticelement or any other type of optical element commonly used for couplinglight into a light guide. A diffractive optical element will in mostcases provide the most compact solution. Substrates 81, 82, the ESBGelement 84 and the transparent region 83 together form a total internalreflection light guiding structure. Patterned ITO electrodes are appliedto the opposing surfaces of the substrates. The transparent region maybe a PDLC region not containing a grating. Desirably the transparentregion 83 would have a refractive substantially the same as that of thesubstrates. The light-coupling optical element provides a means forinjecting light from a laser source, which is not illustrated into thedevice. The light-coupling element may be a DOE. Alternatively thelight-coupling device may be a prismatic element or an array ofprismatic elements. The outer face of substrates 81, that is the faceopposite to the one in contact with the ESBG, provides a light outputsurface. Light emitted from the output surface may be used to illuminatea flat panel display. A complete illumination system will normallyrequire additional elements such as relay lenses, prisms etc which arenot illustrated.

The ESBG element 84 comprises an array of selectively switchable ESBGpixels designed and fabricated according to the principles discussedabove. The ESBG array provides an array of narrow beams or beamlets,separated by small angles. The beamlets have an average angle that issubstantially normal to the plane of the ESBG array. Each ESBG arrayelement may be a plane grating characterised by a unique grating vectoror a grating vector selected from a set of predetermined gratingvectors. The angles of separation of the beamlets are referred to asInter Beamlet Angles (TBAs). In certain embodiments of the inventionsaid ESBG elements might have optical power to control the IBA andindividual beamlet divergence angles. The ESBG elements may encodefurther optical properties. For example, in further embodiments of theinvention the ESBG elements may encode diffusing characteristics foroptimizing the angular extent and uniformity of output light. In yetfurther embodiments of the invention the ESBG elements may encodekeystone correction. The ESBG may be recorded using apparatus similar tothat illustrated in FIG. 32.

Input collimated laser light is indicated by 1750. The light-couplingelement diffracts said input light through a large angle into rays suchas those indicated by 1751,1752 that exceed the substrate-to-aircritical angle as determined by the refractive index of the substrates.As indicated in FIG. 51 rays such as 1751 are diffracted by the ESBGarrays into the direction 1753 substantially normal to the plane of theESBG array. On the other hand rays such as 1752 that propagate throughthe region 83 follow the total internal reflection path generallyindicated by 1752,1754,1755 with the ray 1755 being diffracted by theESBG array into a ray direction indicated by 1756 substantially normalto the plane of the ESBG array. The rays 1752 and 1756 providedespeckled light can be used to illuminate a display panel.

A further embodiment of the invention is shown in the schematic sidleelevation view of the FIG. 52. The embodiment of FIG. 52 differs fromthe embodiment illustrated in FIG. 51 in that the ESBG element is anESBG array 85 containing ESBG pixels such as 85 a. Desirable the ESBGelements have electrodes designed according to the principlesillustrated in FIGS. 21-27. The ray paths indicated by 1760,1761,1763and 1760,1762,1764, 1765,1766 are equivalent to the ray paths1750,1751,1753 and 1750,1752,1754, 1755,1756 respectively illustrated inFIG. 51

A further embodiment of the invention is shown in the schematic sidleelevation view of the of the FIG. 53. The embodiment of FIG. 53 differsfrom the embodiment illustrated in FIG. 51 in that two ESBG layers aresandwiched between the substrates. The ray paths indicated by1770,1771,1773 and 1770,1772,1774,1775,1776 are equivalent to the raypaths 1750,1751,1753 and 1750,1752,1754,1755,1756 respectivelyillustrated in FIG. 51

A further embodiment of the invention is shown in the schematic sidleelevation view of the FIG. 54. The embodiment of FIG. 54 differs fromthe embodiment illustrated in FIG. 51 in that two ESBG layers aresandwiched between the substrates and the ESBG elements are ESBG arrays88,89 containing ESBG pixels such as 88 a,88 b. Desirably the ESBGelements have electrodes designed according to the principlesillustrated in FIGS. 21-27. The ray paths indicated by 1770,1771,1773and 1770,1772,1774, 1775,1776 are equivalent to the ray paths1750,1751,1753 and 1750,1752,1754, 1755,1756 respectively illustrated inFIG. 51

In any of the embodiments illustrate in FIGS. 51-54 a DOE element may beto modify the intensity profile of the illumination light to generate aflat average intensity profile. The DOE element may be disposed at theoutput surface of the substrate 81. A further function of the DOEelement could be to apply a predetermined amount of diffusion to theillumination light or to provide more favorable incident angles to matchthe input light to the diffraction angles of the ESBG elements.

Embodiments Providing Edge Illuminated Despeckler for ColourIllumination

The embodiments of FIGS. 51-54 are directed at monochromatic displays.In a further embodiment of the invention there is provided an edge litcolour despeckler device. Essentially the device provides coloursequential despeckling by stacking red green and blue-diffracting ESBGelements of the type illustrated in FIG. 52. Referring to the schematicside elevation view of FIG. 55 we see that the apparatus comprises firstand second transparent substrates 81,82 three ESBG layers 85A,85B,85Cseparated by transparent spacers 90A,9B sandwiched between thesubstrates, a transparent region 83A,83B,83C adjacent to the ESBGs85A,85B,85C respectively and a light-coupling element 80. Thesubstrates, ESBGs spacers and the transparent regions together form atotal internal reflection light guiding structure. The light-couplingoptical element 80 provides a means for injecting light from a lasersource, which is not illustrated into the device. The light-couplingelement may be a DOE. Alternatively, the light-coupling device may be aprismatic element or an array of prismatic elements. The outer face ofsubstrate 81, that is the face opposite to the one in contact with theESBG provide a light output surface. Light emitted from the outputsurface may be used to illuminate a flat panel display. A completeillumination systems will normally require additional elements such asrelay lenses, prisms etc which are not illustrated. The ray paths foreach colour through the embodiment of FIG. 55 are substantially the sameas those illustrated in FIG. 52 and are not illustrated. It will beclear from consideration of FIG. 55 in relation to FIGS. 51-54 that anyof the embodiments of FIG. 51-54 may be converted to colour sequentialdespeckling device by stacking red green and blue diffracting ESBGsusing the principles illustrated in FIG. 55

Edge Lit Scrolling Despeckler Configurations

The edge lit ESBG despeckler devices discussed above may be configuredto provide a scrolling display panel by patterning the ESBG electrodesto provide a set of selectively switchable ESBG stripes. The basicprinciples of patterning ESBG electrodes for scrolling are disclosed inthe U.S. Provisional Patent Application Ser. No. 61/071,230 filed 18Apr. 2008, entitled SCROLLING ILLUMINATOR and Ser. No. 61/071,229 filed18 Apr. 2008, entitled SCROLLING FLAT PANEL DISPLAY. It is proposed thatthe ESBG despeckler device principles discussed in the presentapplication may be with the scrolling displays concepts disclosed in theabove patent applications to provide a transparent despeckled laserilluminator. In certain cases the above art may be used to provide acomplete integrated display device.

A scrolling illumination scheme for use with the colour displayembodiment of FIG. 55 will now be discussed with reference to theembodiments of FIGS. 55-58.

FIG. 56 shows a schematic plan view of the transparent electrodes withineach of the ESBG layers. The electrodes are divided into a number ofparallel sections or stripes that define a corresponding number ofregions in the gratings that may be independently controlled. Sixteenstripes are shown in this illustrative example, but a different numberof stripes may be used depending on the application. In the exampleshown in FIG. 56, region 1320 of ESBG 851B diffracts blue light.Similarly, region 1330 of ESBG 851G and region 1340 of ESBG 851Rdiffract green and red light, respectively,

FIG. 57 illustrates one operational aspect of the transparent electrodesillustrated in FIG. 56. At an initial instant in time, as shown in FIG.57A, red 1340, green 1330, and blue 1320 bands of light have beendiffracted by means of selecting the appropriate regions of the threeESBG despeckler devices. The regions are selected by applying a suitablevoltage to the transparent electrodes of regions where light extractionis not desired, and not applying a voltage where light extraction isdesired. At a subsequent instant in time, as shown in FIG. 57B, thevoltages applied to the ESBGs have been changed such that the threecolor bands have moved to a lower position. At a third instant in time,as shown in FIG. 58C, the voltages applied to the ESBGs have again beenchanged such that the extreme top 1320A and bottom 1320B portions areilluminated by blue light.

FIG. 58 illustrates another operational aspect of the transparentelectrodes illustrated in FIG. 56. At an initial instant in time, asshown in FIG. 58A, the voltages applied to the ESBGs are selected suchthat green light 1420 is directed at the lower portion display deviceand red slight is directed towards the top portion of the display. At asubsequent instant in time, as shown in FIG. 58B, the transition fromred 1410 to green 1420 has been moved, in sequential steps down thedisplay. At a third instant in time, as shown in FIG. 58C, the voltagesapplied to the ESBGs are selected such that that red light 1410 isdirected at the lower portion and blue light 1430 is directed towardsthe top portion of the ESBGs.

With respect to FIG. 57 and FIG. 58, it should be understood that otherscanning sequences, including scanning multiple color bands, arepossible within the scope of the invention. In all cases, the positionof the color bands are moved in sequential steps by means of selectingthe voltages applied to the three ESBG layers. It should also beunderstood that the switching of the bands must be done in synchronismwith the writing of data to the such that the various areas of thedisplay device display the appropriate information for the respectiveillumination color. Specifically, the information presented on a givenrow must be changed during the time interval after one color ofillumination is removed and before the next illumination color isapplied. Preferably, the different color bands are separated by anon-illuminated dark band to allow time to change the display content.It also must be understood that the entire sequence must be repeated ata sufficient rate that the viewer's eye mergers the sequentialsingle-colored images into a composite full-color image.

Embodiments Providing a Combined Edge Lit Despeckler and Spatial LightModulator

It is known that the diffraction efficiency of an ESBG varies with theapplied voltage. This property allows an ESBG to be used as a lightmodulator. An array of selectively switchable ESBGs may thereforeprovide a spatial light modulator (SLM). The SLM may be used to displayinformation eliminating the need for the separate flat panel displayspecified in the above described embodiments at the same time asproviding the despeckling functions discussed above. Edge Lit colordespeckler and spatial light modulator configurations based on theembodiments of FIGS. 51-55 are illustrated in FIGS. 59-63 respectively.Each of the embodiments of FIGS. 59-63 comprises identical opticalcomponents to those of FIGS. 51-55 respectively. In each case switchingelectronics and connecting wires are generally indicated by 31 and imagegenerator electronics are generally indicated by 32.

Light Guide ESBG Despeckler Using Combined Phase and Angular Diversity

In one embodiment of the invention illustrates in the schematic planview of FIG. 64A and the schematic side elevation view of FIG. 64B thereis provided an edge illuminated despeckler. Referring to FIG. 64B we seethat the edge-illuminated despeckler comprises first and secondtransparent substrates 81,82 and an ESBG element 86 sandwiched betweenthe substrates, an input light-coupling element 87, and an output lightcoupling element 88. The substrates form a total internal reflectionlight guiding structure. A further trapezoidal light element 82 havinginclined surface 83A,83B is disposed adjacent substrate 82. Saidtrapezoidal element may be air separated from substrate 82. PatternedITO electrodes are applied to the opposing surfaces of the substrates.The ESBG comprises separated ESBG regions such as 86A, 86B,86C,86Dsurrounded by clear PDLC a portion of which is indicated by regions suchas the one indicated by 86.

Desirably, the clear PDLC region has a refractive index substantiallythe same as that of the substrates. The input light-coupling opticalelement provides a means for injecting light from a laser source, whichis not illustrated into the device. The output light-coupling opticalelement provides a means for ejecting light from the light guide into anillumination path directed at a flat panel display. At least one of thecoupling elements may be a DOE. Alternatively, at least one of thelight-coupling elements may be a prismatic element or an array ofprismatic elements. A complete illumination system will normally requireadditional elements such as relay lenses, prisms etc which are notillustrated.

Input collimated laser light indicated by 1770 passes through the inputlight-coupling element 87 to provide the light 1771. When the ESBGelement 86A is in an active state and the ESBG element 86B is in aninactive state the light 1771 is diffracted into the direction 1772. Thediffraction angle is designed to exceed the substrate-to-air criticalangle as determined by the refractive index of the substrates. The light1772 is totally internally reflected into the direction 1773 passingthrough the ESBG elements 86B, 86C, 86D in sequence along apredetermined TIR path until said light is ejected from the light guidevia the output coupling device 88. For the purposes of understanding theinvention the output light-coupling device is assumed to be adiffractive optical element. Note that if the substrate 82 and thetrapezoidal element 83 are not air separated the substrate will requirea reflection coating to provide reflection of the light 1772. When theESBG element 86A is not in an active state and the element 86B is in anactive state the light 1771 passes through the ESBG element withoutdeviation providing light 1775. The light 1775 is reflected at thesurfaces 84A to provide the light 1776. The light 1776 is reflected atthe surface 84B to provide the light 1777. The light 1777 is directed atthe ESBG element 86B, which is now in an active state whereupon it isdiffracted into said predetermined TIR path until said light is ejectedfrom the light guide via the output-coupling device 88. Desirably thegrating used to provide the output light coupler has a diffractionefficiency of at least 98%.

In one embodiment of the invention based on FIG. 64 the ESBG elements86A,86B each comprise an array of selectively switchable ESBG elements.The ESBG array provides an array of narrow beams or beamlets, separatedby small angles. The beamlets have an average angle that issubstantially normal to the plane of the ESBG array. Each ESBG arrayelement may be a plane grating characterised by a unique grating vectoror a grating vector selected from a set of predetermined gratingvectors. The angles of separation of the beamlets are referred to asInter Beamlet Angles (IBAs). In certain embodiments of the inventionsaid ESBG elements might have optical power to control the IBA andindividual beamlet divergence angles. The ESBG elements may encodefurther optical properties. For example, in further embodiments of theinvention the ESBG elements may encode diffusing characteristics. In yetfurther embodiments of the invention the ESBG elements may encodekeystone correction. The ESBG may be recorded using apparatus similar tothat illustrated in FIG. 30.

In alternative embodiments of the invention based on FIG. 64 the ESBGelements 86A,86B are configured such that each ESBG elements diffractslight incident at some specified launch angle into a direction normal tothe surfaces of the ESBG elements as indicated in. Essentially, the ESBGelements 86A,86B operate as Hadamard diffusers. Each ESBG 86A,86Bcomprises an array of ESBG pixels that can be switched to provide randompatterns. However, the second ESBG 86B is switched in such a way that itprovides the binary inverse of the pattern provide by the first ESBG86A. Typically the ESBGs are pixelated to provide at least 10×10 arrays.Hence the ESBG elements 86A,86B reduce speckle in the beam according tothe principles of angular diversity

The ESBG elements 86C,86D are configured to operate as variable phaseretarders according to the principles discussed above. For example theESBG elements may be configured as variable sub wavelength gratings.Hence the ESBG elements 86C,86D reduce speckle in the beam according tothe principles of phase or polarization diversity. Advantageously thearray elements of the ESBGs 86A, 86B are offset by a fraction of theESBG element width in at least one of the vertical or horizontal arrayaxes.

The ESBG arrays 86A,86B are driven in a random anti-phase fashion bymeans of an ESBG controller which is not illustrated. FIG. 48C is achart showing voltage versus time applied to the ESBGs 86A and 99B. Toput it another way the ESBGs 86A,86B are operated in tandem withalternating voltages 1001,1002 applied across the ESBG layers. Theoptical effect of each ESBG is varied from zero to maximum value at ahigh frequency by applying an electric field that varies in acorresponding varying fashion. Each incremental change in the appliedvoltage results in a unique speckle phase cell. Referring to FIG. 48C itwill be seen that there is a phase lag between the voltages 1001,1002applied across the ESBGs. The effect of applying such waveforms is thatthe average intensity of the speckle phase cells remains substantiallyconstant, thereby satisfying the statistical requirements for specklereduction. Other types of waveforms may be applied, for examplesinusoidal, triangular, rectangular or other types of regular waveforms.Alternatively, it may be advantageous in statistical terms to usewaveforms based on a random stochastic process such as the waveforms2001,2002 illustrated in the chart of FIG. 48D.

Typically the length of the light guiding structure illustrated in FIG.64 is 11 mm. The thickness of the light guiding structure formed bysubstrate 81,82 as illustrated in FIG. 64B is 2.5 mm. The thickness ofthe trapezoidal light guiding element 83 as illustrated in FIG. 64B is1.1 mm. The diameters of the light input and output coupling elements87, 88 as illustrated in FIG. 64A are each typically 1 mm.

An additional DOE element which is not illustrated may be disposed inthe illumination path near either the input or output light couplingelements to modify the intensity profile of the illumination light togenerate a flat average intensity profile. The DOE element may bedisposed at the output surface of the substrate 81. A further functionof the DOE element could be to apply a predetermined amount of diffusionto the illumination light.

In further embodiments of the invention based on the embodimentillustrated in FIG. 64 may using any of the methods for providingpatterned electrodes discussed above.

In further embodiments of the invention based on the embodimentillustrated in FIG. 64 the ESBG elements may be based on any of thephased diversity and angular diversity speckle reduction methodsdiscussed in the present application.

FIG. 65 is a schematic illustration of an example of an illuminatorusing two ESBG arrays operating in random anti phase. The apparatus ofFIG. 65 is similar to that of FIG. 64. However, in the apparatus of FIG.65 only one beam reflection takes place within the light guidingstructure. Therefore the elements 86D and 86D illustrated in FIG. 64 arenot required. The ESBG 86A,86B are pixelated as shown in FIG. 65C, eachdevice comprising an ESBG array generally indicated by 750 containingpixels such as 751. The aperture defined by the input and output portsrepresented by 87 and 88 is indicated by 752.

It will be clear from the above description of the invention that theESBG despeckler embodiment disclose here may be applied to the reductionof speckle in a wide range of laser displays including front and rearprojection displays, wearable displays, scanned laser beam displays andtransparent displays for use in viewfinders and HUDs.

In preferred practical embodiments of the invention the ESBG layerscontinued in an ESBG despeckler device would be combined in a singleplanar multilayer device. The multilayer ESBG despeckler devices may beconstructed by first fabricating the separate ESBG and then laminatingthe ESBGs using an optical adhesive. Suitable adhesives are availablefrom a number of sources, and techniques for bonding optical componentsare well known. The multilayer structures may also comprise additionaltransparent members, if needed, to control the optical properties of theilluminator.

The advantage of a solid-state approach is the resulting illuminationpatch can be tailored to provide any required shape. Mechanical devicessuch as rotating diffusers would normally only provide a circularillumination patch resulting in significant light loss.

The invention is not limited to any particular type of HPDLC or recipefor fabricating HPDLC. The HPDLC material currently used by theinventors typically switches at 170 us and restores at 320 us. Theinventors believe that with further optimisation the switching times maybe reduced to 140 microseconds.

For the sake of simplicity most of the embodiments of the invention hasbeen discussed with reference to monochromatic illumination. It willclear from the above discussion that the invention may be applied toilluminators using red, green and blue laser sources by providingseparate ESBG layers for each colour.

While the invention has been discussed with reference to single laserdie or rectangular arrays of laser die, it should be emphasized that theprinciples of the invention apply to any configuration of laser die. Theinvention may be used with any type of laser device. For example theinvention may be used with edge-emitting laser diodes, which emitcoherent light or infrared energy parallel to the boundaries between thesemiconductor layers. More recent technologies such as vertical cavitysurface emitting laser (VCSEL) and the Novalux Extended Cavity SurfaceEmitting Laser (NECSEL) emit coherent energy within a cone perpendicularto the boundaries between the layers. The VCSEL emits a narrow, morenearly circular beam than traditional edge emitters, which makes iteasier to extract energy from the device. The NECSEL benefits from aneven narrower emission cone angle. Solid-state lasers emit in theinfrared. Visible wavelengths are obtained by frequency doubling of theoutput. Solid-state lasers may be configured in arrays comprising asmany as thirty to forty individual dies. The laser die are independentlydriven and would normally emit light simultaneously

It should be emphasized that the Figures are exemplary and that thedimensions have been exaggerated. For example thicknesses of the ESBGlayers have been greatly exaggerated.

The ESBGs may be based on any crystal material including nematic andchiral types.

In particular embodiments of the invention any of the ESBG arraysdiscussed above may be implemented using super twisted nematic (STN)liquid crystal materials. STN offers the benefits of pattern diversityand adoption of simpler process technology by eliminating the need forthe dual ITO patterning process described earlier.

The invention may also be used in other applications such as opticaltelecommunications.

Although the invention has been described in relation to what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not limited to thedisclosed arrangements, but rather is intended to cover variousmodifications and equivalent constructions included within the spiritand scope of the invention.

1. An ESBG despeckler device comprising: at least one ESBG element (2;2A,2B; 90A,90B; 28 a,28 b; 99 a,99 b; 3R,4R, 3G,4G, 3B,4B, 90; 94;96;84; 85; 86,87;88,89; 851R,851G,851B; 86A,86B,86C,86D) wherein eachsaid ESBG element is recorded in a Holographic Polymer Dispersed LiquidCrystal sandwiched between transparent substrates to which transparentconductive coatings (130,170; 171;173) have been applied, characterizedin that at least one of said coatings is patterned to provide atwo-dimensional array of independently switchable ESBG pixels (21A,21B;24; 25 a,25 b,25 c; 26 a,26 b,26 c; 27 a,27 b,27 c; 85R,85G,85B; 85 a;88 a,89 a), wherein an electrical control circuit (3;31; 32) isoperative to apply at least first and second voltages across each saidESBG pixel, wherein said ESBG element is disposed along an illuminationpath, wherein each said ESBG pixel is characterized by a first uniquespeckle state under said first applied voltage and a second uniquespeckle state under said second applied voltage, wherein said first andsecond speckle states occur during the integration time of the humaneye.
 2. The apparatus of claim 1 wherein said ESBG despeckler devicecomprises identical first and second ESBG elements and the waveformsapplied to overlapping pixels of said first and second ESBG elementsoperate in anti-phase.
 3. The illumination device of claim 1 whereinsaid first and second voltages are points on a time varying voltagecharacteristic.
 4. The apparatus of claim 1 wherein said ESBG despecklerdevice comprises identical first and second ESBG elements and saidsecond ESBG element is rotated through 180 degrees with respect to saidfirst ESBG element.
 5. The apparatus of claim 1 wherein at least oneESBG element is a hologram of a CGH (256) having a first surface and asecond surface wherein said CGH is designed to convert a laser beamincident at said first surface into a multiplicity of beams from saidsecond surface wherein each beam has a unique direction in space and adiffusion angle, wherein said beams have single point of origin.
 6. Theapparatus of claim 1 wherein at least one ESBG element is recorded usinga CGH having a first surface and a second surface wherein said CGH isdesigned to convert a laser beam incident at said first surface into amultiplicity of beams from said second surface wherein each beam has aunique direction in space and a diffusion angle, wherein said beams havepoints of origin equally spaced around the periphery of said CGH.
 7. Theapparatus of claim 1 wherein each said ESBG pixel is characterized by aunique grating vector.
 8. The apparatus of claim 1 wherein each saidESBG pixel converts incident collimated light into divergent light. 9.The apparatus of claim 1 wherein each said ESBG pixel converts incidentlight into diffuse light.
 10. The apparatus of claim 1 wherein each ESBGpixel has identical optical characteristics.
 11. The apparatus of claim1 wherein said ESBG despeckler device comprises identical first andsecond ESBG elements and ESBG pixels from said first and second ESBGelements substantially overlap in the illumination beam cross section.12. The apparatus of claim 1 wherein said ESBG despeckler devicecomprises identical first and second ESBG elements and ESBG pixels fromsaid first and second ESBG elements are offset by a fraction of the ESBGelement width in at least one of the vertical or horizontal array axesin the illumination beam cross section.
 13. The illumination device ofclaim 1 wherein each said ESBG pixel has a first phase retardingcharacteristic under said first voltage and a second phase retardingcharacteristic under said second voltage.
 14. The illumination device ofclaim 1 wherein each said ESBG pixel has a first light diffusingcharacteristic under said first voltage and a second light diffusingcharacteristic under said second voltage.
 15. The illumination device ofclaim 1 wherein each said ESBG pixel encodes the optical characteristicsof an axicon
 16. The illumination device of claim 1 wherein each saidESBG pixel encodes the optical characteristics of a sub wavelengthgrating phase retarder.
 17. The illumination device of claim 1 whereineach said ESBG pixel encodes the optical characteristics of a diffuser.18. The apparatus of claim 1 wherein said two-dimensional array ofindependently switchable ESBG pixels provides a Hadamard diffuser. 19.The apparatus of claim 1 further comprising a diffractive illuminationprofile shaping element (8).
 20. The apparatus of claim 1 furthercomprising an electrically controllable retarder (46).
 21. The apparatusof claim 1 wherein said despeckler is disposed within an optical traincomprising in series a laser (1;10), a display panel (45; 97) aprojection lens (75,98) and a screen 5).
 22. The apparatus of claim 23further comprising a laser beam expander (7;73,92).
 23. The apparatus ofclaim 23 wherein said ESBG despeckler device is disposed between saidlaser source and said display panel.
 24. The apparatus of claim 23wherein said ESBG despeckler device is disposed between said displaypanel and said projection lens.
 25. The apparatus of claim 1 whereinsaid ESBG element is fabricated using the following steps: providing afirst transparent substrate having a first surface to which an antireflection coating (102) has been applied and a second surface to whicha transparent electrode layer (103) has been applied; removing portionsof said transparent electrode layer to provide a patterned electrodelayer (130) including an first ESBG pixel pad (131); depositing a layerof UV absorbing dielectric material (104) over said patterned electrodelayer; removing the portion of said UV absorbing dielectric materialoverlapping said first ESBG pixel pad; providing a second transparentsubstrate having a first surface to which an anti reflection coating hasbeen applied and a second surface to which a transparent electrode layerhas been applied; removing portions of the transparent electrode layerof said second substrate layer to provide a second patterned electrodelayer (170) including a second ESBG pixel pad (171) substantiallyidentical to and spatially corresponding with said first ESBG pixel pad;combining the substrates to form a display cell with the transparentelectrode coated surfaces of the two substrates aligned in opposingdirections and having a small separation, wherein the antireflectioncoated surface of said first substrate forms a first cell face, whereinthe antireflection coated surface of said second substrate forms asecond cell face; filling said display cell with a PDLL mixture;illuminating said first cell face by crossed UV laser beams, andsimultaneously illuminating said second cell face formed by anincoherent UV source, wherein said step of illuminating said first cellface by crossed UV laser beams, and simultaneously illuminating saidsecond cell face formed by an incoherent UV source forms an ESBGconfined to the region between said first symbol pad and said secondESBG pixel pad.